Hempel, "Problems and Changes in the Empiricist Criterion of
Meaning"
[posted March 21, 1997]
[Original pagination shown in bold brackets; footnotes have been
reformatted]
"Problems and Changes in the Empiricist Criterion of Meaning"
by Carl G. Hempel 11 Rev. Intern. de Philos. 41 (1950), pages
41-63
[page 41:]
1. Introduction
The fundamental
tenet of modern empiricism is the view that all
non-analytic knowledge is based on experience. Let us call this thesis the
principle of empiricism.[1]
Contemporary logical empiricism has added[2] to it the maxim that a sentence
makes a cognitively meaningful assertion, and thus can be said to be either
true or false, only if it is either (1) analytic or self-contradictory or (2)
capable, at least in principle, of experiential test. According to this
so-called empiricist criterion of cognitive meaning, or of cognitive
significance, many of the formulations of traditional metaphysics and large
parts of epistemology are devoid of cognitive significance--however rich some
of them may be in non-cognitive import by virtue of their emotive appeal or the
moral inspiration they offer. Similarly certain doctrines which have been, at
one time or another, formulated within empirical science or its border
disciplines are so contrived as to be incapable of test by any conceivable
evidence; they are therefore qualified as pseudo- [p. 42:] hypotheses,
which assert nothing, and which therefore have no explanatory or predictive
force whatever. This verdict applies, for example, to the neo-vitalist
speculations about entelechies or vital forces, and to the "telefinalist
hypothesis" propounded by Lecomte du Noüy.[3]
The preceding formulations of the principle of empiricism and of the empiricist
meaning criterion provide no more, however, than a general and rather vague
characterization of a basic point of view, and they need therefore to be
elucidated and amplified. And while in the earlier phases of its development,
logical empiricism was to a large extent preoccupied with a critique of
philosophic and scientific formulations by means of those fundamental
principles, there has been in recent years an increasing concern with the
positive tasks of analyzing in detail the logic and methodology of empirical
science and of clarifying and restating the basic ideas of empiricism in the
light of the insights thus obtained. In the present article, I propose to
discuss some of the problems this search has raised and some of the results it
seems to have established.
2. Changes in the testability criterion of empirical meaning
As our formulation
shows, the empiricist meaning criterion lays
down the requirement of experiential testability for those among the
cognitively meaningful sentences which are neither analytic nor contradictory;
let us call them sentences with empirical meaning, or empirical significance.
The concept of testability, which is to render precise the vague notion of
being based--or rather baseable--on experience, has undergone several
modifications which reflect an increasingly refined analysis of the structure
of empirical knowledge. In the present section, let us examine the major
stages of this development.
For convenience of exposition, we first introduce three auxiliary concepts,
namely those of observation characteristic, of observation predicate, and of
observation sentence. A property or a relation of physical objects will be
called an observable characteristic if, under suitable circumstances, its
presence or absence in a given instance can be ascertained [p. 43]
through direct observation. Thus, the terms "green", "soft", "liquid",
"longer than", designate observable characteristics, while "bivalent",
"radioactive", "better electric conductor", and "introvert" do not. Terms
which designate observable characteristics will be called observation
predicates. Finally, by an observation sentence we shall understand any
sentence which--correctly or incorrectly--asserts of one or more specifically
named objects that they have, or that they lack, some specified observable
characteristic. The following sentences, for example, meet this condition:
"The Eiffel Tower is taller than the buildings in its vicinity", "The pointer
of this instrument does not cover the point marked `3' on the scale", and even,
"The largest dinosaur on exhibit in New York's Museum of Natural History had a
blue tongue"; for this last sentence assigns to a specified object a
characteristic-having a blue tongue-which is of such a kind that under suitable
circumstances (e.g., in the case of my Chow dog) its presence or absence can be
ascertained by direct observation. Our concept of observation sentence is
intended to provide a precise interpretation of the vague idea of a sentence
asserting something that is "in principle" ascertainable by direct observation,
even though it may happen to be actually incapable of being observed by myself,
perhaps also by my contemporaries, and possibly even by any human being who
ever lived or will live. Any evidence that might be adduced in the test of an
empirical hypothesis may now be thought of as being expressed in observation
sentences of this kind.[4]
We now turn to the changes in the conception of testability, and thus of
empirical meaning. In the early days of the Vienna Circle, a sentence was said
to have empirical [p 44:] meaning if it was capable, at least in
principle, of complete verification by observational evidence; i.e., if
observational evidence could be described which, if actually obtained, would
conclusively establish the truth of the sentence.[5] With the help of the concept of
observation sentence, we can restate this requirement as follows: A sentence S
has empirical meaning if and only if it is possible to indicate a finite set of
observation sentences, O1, O2, ..., On, such that if these, are true, then S is
necessarily true, too. As stated, however, this condition is satisfied also if
S is an analytic sentence or if the given observation sentences are logically
incompatible with each other. By the following formulation, we rule these
cases out and at the same time express the intended criterion more precisely:
[p. 45:]
(2.1) Requirement of complete verifiability in principle:
A
sentence has empirical meaning if and only if it is not analytic and follows
logically from some finite and logically consistent class of observation
sentence.[6]
This criterion, however, has several serious defects. The [p. 46:]
first of those here to be mentioned has been pointed out by various
writers:
(a) The verifiability requirement rules out all sentences of universal form
and thus all statements purporting to express general laws; for these cannot be
conclusively verified by any finite set of observational data. And since
sentences of this type constitute an integral part of scientific theories, the
verifiability requirement must be regarded as overly restrictive in this
respect. Similarly, the criterion disqualifies all sentences such as "For any
substance there exists some solvent", which contain both universal and
existential quantifiers (i.e., occurrences of the terms "all" and "some" or
their equivalents); for no sentences of this kind can be logically deduced from
any finite set of observation sentences.
Two further
defects of the verifiability requirement do not seem to have been
widely noticed:
(b) Suppose that S is a sentence which satisfies the proposed criterion,
whereas N is a sentence such as "The absolute is perfect", to which the
criterion attributes no empirical meaning. Then the alternation S v N (i.e.,
the expression obtained by connecting the two sentences by the word "or"),
likewise satisfies the criterion; for if S is a consequence of some finite
class of observation sentences, then trivially S v N is a consequence of the
same class. But clearly, the empiricist criterion of meaning is not intended
to countenance sentences of this sort. In this respect, therefore, the
requirement of complete verifiability is too inclusive.
(c) Let "P" be an observation predicate. Then the purely existential sentence
"(Ex)P(x)" ("There exists at least one thing that has the property P") is
completely verifiable, for it follows from any observation sentence (asserting
of some particular object that it has the property P. But its denial, being
equivalent to the universal sentence "(x)~P(x)" ("Nothing has the
property P") is clearly not completely verifiable, as follows from comment (a)
above. Hence, under the criterion (2.1), the denials of certain
empirically--and thus cognitively-significant sentences--are empirically
meaningless; and as they are neither analytic nor contradictory, they are
cognitively meaningless. But however we may [p. 47:] delimit the domain
of significant discourse, we shall have to insist that if a sentence falls
within that domain, then so must its denial. To put the matter more
explicitly: The sentences to be qualified as cognitively meaningful are
precisely those which can be significantly said to be either true or false.
But then, adherence to (2.1) would engender a serious dilemma, as is shown by
the consequence just mentioned: We would either have to give up the fundamental
logical principle that if a sentence is true or false, then its denial is false
or true, respectively (and thus cognitively significant); or else, we must
deny, in a manner reminiscent of the intuitionistic conception of logic and
mathematics, that "(x) ~ P(x)" is logically equivalent to the negation of
"(Ex)P(x)". Clearly, the criterion (2.1), which has disqualified itself on
several other counts, does not warrant such drastic measures for its
preservation; hence, it has to be abandoned.[7]
Strictly analogous considerations apply to an alternative criterion, which
makes complete falsifiability in principle the defining characteristic of
empirical significance. Let us formulate this criterion as follows: A sentence
has empirical meaning if and only if it is capable, in principle, of complete
refutation by a finite number of observational data; or, more precisely:
(2.2) Requirement of complete falsifiability in principle: A sentence
has empirical meaning if and only if its denial is [p. 48:] not analytic
and follows logically from some finite logically consistent class of
observation sentences.[8]
This criterion qualifies a sentence as empirically meaningful if its denial
satisfies the requirement of complete verifiability; as is to be expected, it
is therefore inadequate on similar grounds as the latter:
(a) It rules out purely existential hypotheses, such as "There exists at least
one unicorn", and all sentences whose formulation calls for mixed--i.e.,
universal and existential--quantification; for none of these can possibly be
conclusively falsified by a finite number of observation sentences.
(b) If a sentence S is completely falsifiable whereas N is a sentence which is
not, then their conjunction, S·N (i.e., the expression obtained by connecting
the two sentences by the word "and") is completely falsifiable; for if the
denial of S is entailed by some class of observation sentences, then the denial
of S·N is, a fortiori, entailed by the same class. Thus, the criterion allows
empirical significance to many sentences which an adequate empiricist criterion
should rule out, such as, say "All swans are white and the absolute is
perfect."
(c) If "P" is an observation predicate, then the assertion that all things
have the property P is qualified as significant, but its denial, being
equivalent to a purely existential hypothesis, is disqualified (cf. (a)).
Hence, criterion (2.2) gives rise to the same dilemma as (2.1).
In sum, then, interpretations of the testability criterion in terms of complete
verifiability or of complete falsifiability are inadequate because they are
overly restrictive in one direction and overly inclusive in another, and
because both of them require incisive changes in the fundamental principles of
logic.
Several attempts
have been made to avoid these difficulties by construing the
testability criterion as demanding merely a partial and possibly indirect
confirmability of empirical hypotheses by observational evidence. [p 49:]
(2.3) A formulation suggested by Ayer[9] is characteristic of these attempts
to set up a clear and sufficiently comprehensive criterion of confirmability.
It states, in effect, that a sentence S has empirical import if from S in
conjunction with suitable subsidiary hypotheses it is possible to derive
observation sentences which are not derivable from the subsidiary hypotheses
alone.
This condition is
suggested by a closer consideration of the logical structure
of scientific testing; but it is much too liberal as it stands. Indeed, as
Ayer himself has pointed out in the second edition of his book, Language,
Truth, and Logic,[10] his criterion
allows empirical import to any sentence whatever. Thus, e.g., if S is the
sentence "The absolute is perfect", it suffices to choose as a subsidiary
hypothesis the sentence "If the absolute is perfect then this apple is red" in
order to make possible the deduction of the observation sentence "This apple is
red," which clearly does not follow from the subsidiary hypothesis alone.[11] [p 50:]
(2.4) To meet this objection, Ayer has recently proposed a modified version of
his testability criterion. The modification restricts, in effect, the
subsidiary hypotheses mentioned in (2.3) to sentences which are either analytic
or can independently be shown to be testable in the sense of the modified
criterion.[12]
But it can readily be shown that this new criterion, like the requirement
complete falsifiability, allows empirical significance to any conjunction
S·N, where S satisfies Ayer's criterion A, while N is a sentence such as
"The absolute is perfect," which is to be disqualified by that criterion.
Indeed: whatever consequences can be deduced from S with the help of
permissible subsidiary hypotheses can also be deduced from S·N by means
of the same subsidiary hypotheses, and as Ayer's new criterion is formulated
essentially in terms of the deducibility of a certain type of consequence from
the given sentence, it countenances S·N together with S. Another
difficulty has been pointed out by Professor A. Church, who has shown[13] that if there are any three
observation sentences none of which alone entails any of the others, then it
follows for any sentence S whatsoever that either it or its denial has
empirical import according to Ayer's revised criterion.
3. Translatability into an empiricist language as a new criterion of
cognitive meaning
I think it is useless
to continue the search for an adequate criterion
of testability in terms of deductive relationships to observation sentences.
The past development of this search--of which we have considered the major
stages--seems to warrant the expectation that as long as we try to set up a
criterion of testability for individual sentences in a natural language, in
terms of logical relationship to observation sentences, the result will be
either too restrictive or too inclusive, [p. 51:] or both. In
particular it appears likely that such criteria would allow empirical import,
in the manner of (2.1) (b) or of (2.2) (b), either to any
alternation or to any conjunction of two sentences of which at least one is
qualified as empirically meaningful; and this peculiarity has undesirable
consequences because the liberal grammatical rules of English as of any other
natural language countenance as sentences certain expressions ("The absolute is
perfect" was our illustration) which even by the most liberal empiricist
standards make no assertion whatever; and these would then have to be permitted
as components of empirically significant statements.
The predicament would not arise, of course, in an artificial language whose
vocabulary and grammar were so chosen as to preclude altogether the possibility
of forming sentences of any kind which the empiricist meaning criterion is
intended to rule out. Let us call any such language an empiricist language.
This reflection suggests an entirely different approach to our problem: Give a
general characterization of the kind of language that would qualify as
empiricist, and then lay down the following
(3.1) Translatability criterion of cognitive meaning: A sentence has cognitive
meaning if and only if it is translatable into an empiricist language.
This conception of cognitive import, while perhaps not explicitly stated, seems
to underlie much of the more recent work done by empiricist writers; as far as
I can see it has its origin in Carnap's essay, Testability and Meaning
(especially part IV).
As any language, so also any empiricist language can be characterized by
indicating its vocabulary and the rules determining its logic; the latter
include the syntactical rules according to which sentences may be formed by
means of the given vocabulary. In effect, therefore, the translatability
criterion proposes to characterize the cognitively meaningful sentences by the
vocabulary out of which they may be constructed, and by the syntactical
principles governing their construction. What sentences are singled out as
cognitively significant will depend, accordingly, on the choice of the
vocabulary and of the construction rules. Let us consider a specific
possibility: [p. 52:]
(3.2) We might qualify a language L as empiricist if it satisfies the
following conditions:
(a) The vocabulary of L contains:
(1) The customary locutions of logic which are used in the formulation of
sentences; including in particular the expressions "not", "and", "or", "if...
then...", "all", "some", "the class of all things such that...", "... is an
element of class...";
(2) Certain observation predicates. These will be said to constitute the
basic empirical vocabulary of L;
(3) Any expression definable by means of those referred to under (1) and
(2).
(b) The rules of sentence formation for L are those laid down in some
contemporary logical system such as Principia Mathematica.
Since all defined terms can be eliminated in favor of primitives, these rules
stipulate in effect that a language L is empiricist if all its sentences are
expressible, with the help of the usual logical locutions, in terms of
observable characteristics of physical objects. Let us call any language of
this sort a thing-language in the narrower sense. Alternatively, the basic
empirical vocabulary of an empiricist language might be construed as consisting
of phenomenalistic terms, each of them referring to some aspect of the
phenomena of perception or sensation. The construction of adequate
phenomenalistic languages, however, presents considerable difficulties[14], and in recent empiricism,
attention has been focussed primarily on the potentialities of languages whose
basic empirical vocabulary consists of observation predicates; for the latter
lend themselves more directly to the description of that type of
intersubjective evidence which is invoked in the test of scientific
hypotheses.
If we construe empiricist languages in the sense of (3.2), then the
translatability criterion (3.1) avoids all of the short-comings pointed out in
our discussion of earlier forms of the testability criterion [p. 53:]
(a) Our characterization of empiricist languages makes explicit
provision for universal and existential quantification, i.e., for the use of
the terms "all" and "some"; hence, no type of quantified statement is generally
excluded from the realm of cognitively significant discourse;
(b) Sentences such as "The absolute is perfect" cannot be formulated in an
empiricist language (cf. (d) below); hence there is no danger that a
conjunction or alternation containing a sentence of that kind as a component
might be qualified as cognitively significant;
(c) In a language L with syntactical rules conforming to Principia
Mathematica, the denial of a sentence is always again a sentence of L.
Hence, the translatability criterion does not lead to the consequence, which is
entailed by both (2.1) and (2.2), that the denials of certain significant
sentences are non-significant;
(d) Despite its comprehensiveness, the new criterion does not attribute
cognitive meaning to all sentences; thus, e.g., the sentences "The absolute is
perfect" and "Nothingness nothings" cannot be translated into an empiricist
language because their key terms are not definable by means of purely logical
expressions and observation terms.
4. The problem of disposition terms and of theoretical
constructs
Yet, the new
criterion is still too restrictive--as are,
incidentally, also its predecessors--in an important respect which now calls
for consideration. If empiricist languages are defined in accordance with
(3.2), then, as was noted above, the translatability criterion (3.1) allows
cognitive import to a sentence only if its constitutive empirical terms are
explicitly definable by means of observation predicates. But as we shall argue
presently, many terms even of the physical sciences are not so definable; hence
the criterion would oblige us to reject, as devoid of cognitive import, all
scientific hypotheses containing such terms--an altogether intolerable
consequence.
The concept of temperature is a case in point. At first glance, it seems as
though the phrase "Object x has a temperature of c degrees centigrade", or
briefly "T(x)=c" could [p. 54:] be defined by the following sentence,
(D): T(x)=c if and only if the following condition is satisfied: If a
thermometer is in contact with x, then it registers c degrees on its scale.
Disregarding niceties, it may be granted that the definiens given here is
formulated entirely in reference to observables. However, it has one highly
questionable aspect: In Principia Mathematica and similar systems, the
premise "if p then q" is construed as being synonymous with "not p or q"; and
under this so-called material interpretation of the conditional, a statement of
the form "if p then q" is obviously true if (though not only if) the sentence
standing in the place of "p" is false. If, therefore, the meaning of "if ...
then..." in the definiens of (D) is understood in the material sense, then that
definiens is true if (though not only if) x is an object not in contact with a
thermometer--no matter what numerical value we may give to c. And since the
definiendum would be true under the same circumstances, the definition (D)
would qualify as true the assignment of temperature value whatsoever to any
object not in contact with a thermometer! Analogous considerations apply to
such terms as electrically charged", "magnetic", "intelligent", "electric
resistance", etc., in short to all disposition terms, i.e., terms which express
the disposition of one or more objects to react in a determinate way under
specified circumstances: A definition of such terms by means of observation
predicates can not be effected in the manner of (D), however natural and
obvious a mode of definition this may it first seem to be.[15]
There are two main directions in which a resolution of the difficulty might be
sought. On the one hand, it could be argued the definition of disposition
terms in the manner of (D) is perfectly adequate provided that the phrase
"if...then..." in the definiens is construed in the sense it is obviously
intended to have, namely as implying, in the else of (D), that even if x is not
actually in contact with a thermometer, still if it were in such contact, then
the thermometer would register c degrees. In sentences such as this, the
phrase "if...then..." is said to be used counterfactually; and it is in this
strong" sense, which implies a counterfactual conditional, [p. 55:] that
the definiens of (D) would have to be construed. This suggestion would provide
an answer to the problem of defining disposition terms if it were not for the
fact that no entirely satisfactory account of the exact meaning of
counterfactual conditionals seems to be available at present. Thus, the first
way out of the difficulty has the status of a program rather than that of a
solution. The lack of an adequate theory of counterfactual conditionals is all
the more deplorable as such a theory is needed also for the analysis of the
concept of general law in empirical science and of certain related ideas. A
clarification of this cluster of problems constitutes at present one of the
urgent desiderata in the logic and methodology of science.[16]
An alternative way of dealing with the definitional problems raised by
disposition terms was suggested, and developed in detail, by Carnap. It
consists in permitting the introduction of new terms, within an empiricist
language, by means of so-called reduction sentences, which have the character
of partial or conditional definitions.[17] Thus, e.g., the concept of
temperature in our last illustration might be introduced by means of the
following reduction sentence, (R): If a thermometer is in contact with an
object x, then T(x)=c if and only if the thermometer registers c degrees.
This rule, in which the conditional may be construed in the material sense,
specifies the meaning of "temperature", i.e., of statements of the form
"T(x)=c", only partially, namely in regard to those objects which are in
contact with a thermometer; for all other objects, it simply leaves the meaning
of "T(x)=c" undetermined. The specification of the meaning [p. 56:] of
"temperature" may then be gradually extended to cases not covered in (R) (b)
laying down further reduction sentences, which reflect the measurement of
temperature by devices other than thermometers.
Reduction sentences thus provide a means for the precise formulation of what is
commonly referred to as operational definitions.[18] At the same time, they show that
the latter are not definitions in the strict sense of the word, but rather
partial specifications of meaning.
The preceding considerations suggest that in our characterization (3.2) of
empiricist languages we broaden the provision a (3) by permitting in the
vocabulary of L all those terms whose meaning can be specified in terms of the
basic empirical vocabulary by means of definitions or reduction sentences.
Languages satisfying this more inclusive criterion will be referred to as
thing-languages in the wider sense.
If the concept of empiricist language is broadened in this manner, then the
criterion (3.1) covers--as it should--also all those statements
whose constituent empirical terms include "empirical constructs", i.e., terms
which do not designate observables, but which can be introduced by reduction
sentences on the basis of observation predicates. Even in this generalized
version, however, our criterion of cognitive meaning may not do justice to
advanced scientific theories, which are formulated in terms of "theoretical
constructs", such as the terms "absolute temperature", "gravitational
potential", "electric field", "[psi]-function", etc. There are reasons to
think that neither definitions nor reduction sentences are, adequate to
introduce these terms on the basis of observation predicates. Thus, e.g., if a
system of reduction sentences for the concept of electric field were available,
then--to oversimplify the point a little--it would be possible to describe, in
term of observable characteristics, some necessary and some sufficient
conditions for the presence, in any given region, of an electric field of any
mathematical description, however complex. Actually, however, such criteria
can at best be given only for some sufficiently simple kinds of fields. [p.
57:]
Now theories of
the advanced type here referred to may be considered as
hypothetico-deductive systems in which all statements are logical consequences
of a set of fundamental assumptions. Fundamental as well as derived statements
in such a system are formulated either in terms of certain theoretical
constructs which are not defined within the system and thus play the role of
primitives, or in terms of expressions defined by means of the latter. Thus,
in their logical structure such systems equal the axiomatized uninterpreted
systems studied in mathematics and logic. They acquire applicability to
empirical subject matter, and thus the status of theories of empirical science,
by virtue of an empirical interpretation. The latter is effected by a
translation of some of the sentences of the theory--often derived rather than
fundamental ones--into an empiricist language, which may contain both
observation predicates and empirical constructs. And since the sentences which
are thus given empirical meaning are logical consequences of the fundamental
hypotheses of the theory, that translation effects, indirectly, a partial
interpretation of the latter and of the constructs in terms of which they are
formulated.[19]
In order to make translatability into an empiricist language an adequate
criterion of cognitive import, we broaden therefore the concept of empiricist
language so as to include thing-languages in the narrower and in the wider
sense as well as all interpreted theoretical systems of the kind just referred
to.[20] With this understanding,
(3.1) may finally serve as a general criterion of cognitive meaning. [p.
58:]
5. On "the meaning" of an empirical statement
In effect, the
criterion thus arrived at qualifies a sentence as
cognitively meaningful if its non-logical constituents refer, directly or in
certain specified indirect ways, to observables. But it does not make any
pronouncement on what "the meaning" of a cognitively significant sentence is,
and in particular it neither says nor implies that that meaning can be
exhaustively characterized by what the totality of possible tests would reveal
in terms of observable phenomena. Indeed, the content of a statement with
empirical import cannot, in general, be exhaustively expressed by means of any
class of observation sentences.
For consider first, among the statements permitted by our criterion, any purely
existential hypothesis or any statement involving mixed quantification. As was
pointed out earlier, under (2.2) (a), statements of these kinds entail
no observation sentences whatever; hence their content cannot be expressed by
means of a class of observation sentences.
And secondly, even most statements of purely universal form (such as "All
flamingoes are pink") entail observation sentences (such as "That thing is
pink") only when combined with suitable observation sentences (such as "That
thing is a flamingo").
This last remark can be generalized: The use of empirical hypotheses for the
prediction of observable phenomena requires, in practically all cases, the use
of subsidiary empirical hypotheses.[21] Thus, e.g., the hypothesis that
the agent of tuberculosis is rod-shaped does not by itself entail the
consequence that upon looking at a tubercular sputum specimen through a
microscope, rod-like shapes will be observed: a large number of subsidiary
hypotheses, including the theory [p. 59:] of the microscope, have to be
used as additional premises in deducing that prediction.
Hence, what is sweepingly referred to as "the (cognitive) meaning" of a given
scientific hypothesis cannot be adequately characterized in terms of potential
observational evidence alone, nor can it be specified for the hypothesis taken
in isolation: In order to understand "the meaning" of a hypothesis within an
empiricist language, we have to know not merely what observation sentences it
entails alone or in conjunction with subsidiary hypotheses, but also what
other, non-observational, empirical sentences are entailed by it, what
sentences in the given language would confirm or disconfirm it, and for what
other hypotheses the given one would be confirmatory or disconfirmatory. In
other words, the cognitive meaning of a statement in an empiricist language is
reflected in the totality of its logical relationships to all other statements
in that language and not to the observation sentences alone. In this sense,
the statements of empirical science have a surplus meaning over and above what
can be expressed in terms of relevant observation sentences.[22]
6. The logical status of the empiricist criterion of meaning
What kind of a
sentence, it has often been asked, is the
empiricist meaning criterion itself? Plainly it is not an empirical
hypothesis; but it is not analytic or self-contradictory either; hence, when
judged by its own standard, is it not devoid of cognitive meaning? In that
case, what claim of soundness or validity could possibly be made for it?
One might think of construing the criterion as a definition which indicates
what empiricists propose to understand by a cognitively significant sentence;
thus understood, it would not have the character of an assertion and would be
neither true nor false. But this conception would attribute to the criterion a
measure of arbitrariness which cannot be reconciled with the heated
controversies it has engendered and even less with the fact, repeatedly
illustrated in the present article, that the changes in its specific content
have always been [p. 60:] determined by the objective of making the
criterion a more adequate index of cognitive import. And this very objective
illuminates the character of the empiricist criterion of meaning: It is
intended to provide a clarification and explication of the idea of a sentence
which makes an intelligible assertion.[23] This idea is admittedly vague,
and it is the task of philosophic explication to replace it by a more precise
concept. In view of this difference of precision we cannot demand, of course,
that the "new" concept, the explicatum, be strictly synonymous with the old
one, the explicandum.[24] How,
then, are we to judge the adequacy of a proposed explication, as expressed in
some specific criterion of cognitive meaning?
First of all, there exists a large class of sentences which are rather
generally recognized as making intelligible assertions, and another large class
of which this is more or less generally denies. We shall have to demand of an
adequate explication that it take into account these spheres of common usage;
hence an explication which, let us say, denies cognitive import to descriptions
of past events or to generalizations expressed in terms of observables has to
be rejected as inadequate. As we have seen, this first requirement of adequacy
has played an important role in the development of the empiricist meaning
criterion.
But an adequate explication of the concept of cognitively significant statement
must satisfy yet another, even more important, requirement: Together with the
explication of certain other concepts, such as those of confirmation and of
probability, it has to provide the framework for a general theoretical account
of the structure and the foundations of scientific knowledge. Explication, as
here understood, is not a mere description of the accepted usages of the terms
under [p. 61:] consideration: it has to go beyond the limitations,
ambiguities, and inconsistencies of common usage and has to show how we had
better construe the meanings of those terms if we wish to arrive at a
consistent and comprehensive theory of knowledge. This type of consideration,
which has been largely influenced by a study of the structure of scientific
theories, has prompted the more recent extensions of the empiricist meaning
criterion. These extensions are designed to include in the realm of cognitive
significance various types of sentences which might occur in advanced
scientific theories, or which have to he admitted simply for the sake of
systematic simplicity and uniformity,[25] but on whose cognitive
significance or non-significance a study of what the term "intelligible
assertion" means in everyday discourse could hardly shed any light at all.
As a consequence, the empiricist criterion of meaning, like the result of any
other explication, represents a linguistic proposal which itself is neither
true nor false, but for which adequacy is claimed in two respects: First in the
sense that the explication provides a reasonably close analysis of the commonly
accepted meaning of the explicandum--and this claim implies an empirical
assertion; and secondly in the sense that the explication achieves a "rational
reconstruction" of the explicandum, i.e., that it provides, together perhaps
with other explications, a general conceptual framework which permits a
consistent and precise restatement and theoretical systematization of the
contexts in which the explicandum is used--and this claim implies at least an
assertion of a logical character.
Though a proposal in form, the empiricist criterion of meaning is therefore far
from being an arbitrary definition; it is subject to revision if a violation of
the requirements of adequacy, or even a way of satisfying those requirements
more fully, should be discovered. Indeed, it is to be hoped that [p.62:]
before long some of the open problems encountered in the analysis of
cognitive significance will be clarified and that then our last version of the
empiricist meaning criterion will be replaced by another, more adequate one.
Yale University.
FOOTNOTES to Hempel, "Problems and Changes in an Empiricist Criterion of Meaning"
[1] This term is used by Benjamin, A. C., Is empiricism self-refuting? (Journal of
Philos., vol. 38, 1941),
in an examination of the foundations of empiricism. For a recent discussion of
the basic ideas of empiricism see Russell, B., Human Knowledge, Simon and Schuster, New York, 1948, Part Six.
[2] In his stimulating article, Positivism, W. T. Stace
argues, in effect, that the testability criterion of meaning is not logically
entailed by the principle of empiricism. (See STACE, W. T., Positivism (Mind, vol. 53, 1944), especially section 11.)
This is correct: According to the latter, a sentence expresses knowledge only
if it is either analytic or corroborated by empirical evidence; the former goes
further and identifies the domain of cognitively significant discourse with
that of potential knowledge; i. e., it grants cognitive import only to
sentences for which--unless they are either analytic or contradictory--a test
by empirical evidence is conceivable.
[3] Cf. LECOMTE Du NOÜY, Human Destiny, New York, London, Toronto,
1947, Ch. XVI.
[4] Observation sentences of this kind belong to what Carnap
has called the thing-language (cf., e. g., CARNAP, R., Logical foundations of the unity of science, In:
Internat. Encyclopedia of Unified Science, 1, 1; Univ. of Chicago Press,
1938, pp. 52-53). That they are
adequate to formulate the data which serve as the basis for empirical tests is
clear in particular for the intersubjective testing procedures used in science
as well as in large areas of empirical inquiry on the common-sense level. In
epistemological discussions, it is frequently assumed that the ultimate
evidence for beliefs about empirical matters consists in perceptions and
sensations whose description calls for a phenomenalistic type of language. The
specific problems connected with the phenomenalistic approach cannot be
discussed here; but it should be mentioned that at any rate all the critical
consideration presented in this article in regard to the testability criterion
are applicable, mutatis mutandis, to the case of a phenomenalistic basis as
well.
[5] Originally, the permissible evidence was meant to be
restricted to what is observable by the speaker and perhaps his fellow-beings
during their life times. Thus construed, the criterion rules out, as
cognitively meaningless, all statements about the distant future or the remote
past, as has been pointed out, among others, by Ayer, A. J., Language, Truth and Logic, Oxford Univ. Press, 1936;
2nd ed., Gollancz, London, 1946., Chapter 1; by Pap, A., Elements of Analytic Philosophy, The Macmillan Co., New
York, 1949, Chapter 13, esp. pp. 333 ff.; and by Russell, B., Human Knowledge, Simon and Schuster, New York, 1948, pp. 445-447.
This difficulty is avoided, however, if we permit the evidence to consist of
any finite set of "logically possible observation data", each of them
formulated in an observation sentence. Thus, e. g., the sentence S1, ("The
tongue of the largest dinosaur in New York's Museum of Natural History was blue
or black" is completely verifiable in our sense; for it is a logical
consequence of the sentence S2, "The tongue of the largest dinosaur in New
York's Museum of Natural History was blue"; and this is an observation
sentence, as has been shown above.
And if the concept of verifiability in principle and the more general concept
of confirmability in principle, which will be considered later, are construed
as referring to logically possible evidence as expressed by observation
sentences, then it follows similarly that the class of statements which are
verifiable, or at least confirmable, in principle includes such assertions as
that the planet Neptune and the Antarctic Continent existed before they were
discovered, and that atomic warfare, if not checked, may lead to the
extermination of this planet. The objections which Russell (cf. RUSSELL, B., Human Knowledge, Simon and Schuster, New York, 1948, pp. 445
and 447) raises against the verifiability criterion by reference to those
examples do not apply therefore if the criterion is understood in the manner
here suggested. Incidentally, statements of the kind mentioned by Russell,
which are not actually verifiable by any human being, were explicitly
recognized as cognitively significant already by Schlick (in SCHLICK, M., Meaning and Verification (Philos. Review, vol.
45,1936). (Also reprinted in Feigl and Sellars, Readings in Philosophical
Analysis, New York, 1949), Part V), who
argued that the impossibility of verifying them was "merely empirical". The
characterization of verifiability with the help of the concept of observation
sentence as suggested here might serve as a more explicit and rigorous
statement of that conception.
[6] As has frequently been emphasized in empiricist
literature, the term "verifiability" is to indicate, of course, the
conceivability, or better, the logical possibility of evidence of an
observational kind which, if actually encountered, would constitute conclusive
evidence for the given sentence; it is not intended to mean the technical
possibility of performing the tests needed to obtain such evidence, and even
less does it mean the possibility of actually finding directly observable
phenomena which constitute conclusive evidence for that sentence--which would
be tantamount to the actual existence of such evidence and would thus imply the
truth of the given sentence. Analogous remarks apply to the terms
"falsifiability" and "confirmability". This point has been disregarded in some
recent critical discussions of the verifiability criterion. Thus, c. g.,
Russell (cf. RUSSELL, B., Human Knowledge, Simon and Schuster, New York, 1948, p. 448) construes verifiability as the actual existence of a
set of conclusively verifying occurrences. This conception, which has never
been advocated by any logical empiricist, must naturally turn out to be
inadequate since according to it the empirical meaningfulness of a sentence
could not be established without gathering empirical evidence, and moreover
enough of it to permit a conclusive proof of the sentences in question! It is
not surprising, therefore, that his extraordinary interpretation of
verifiability leads Russell to the conclusion: "In fact, that a proposition is
verifiable is itself not verifiable" (l[oc]. c[it].) Actually, under the
empiricist interpretation of complete verifiability, any statement asserting
the verifiability of some sentence S whose text is quoted, is either analytic
or contradictory; for the decision whether there exists a class of observation
sentences which entail S, i. e., whether such observation sentences can be
formulated, no matter whether they are true or false--that decision is a matter
of pure logic and requires no factual information whatever.
A similar misunderstanding is in evidence in the following passage in which W.
H. Werkmeister claims to characterize a view held by logical positivists : "A
proposition is said to he `true' when it is .verifiable in principle'; i. e.,
when we know the conditions which, when realized, will make `verification'
possible (cf. Ayer)." (cf. WERKMEISTER, W. H., The Basis and Structure of Knowledge, Harper,
New York and London, 1948], p. 145). The quoted thesis, which, again, was
never held by any logical positivist, including Ayer, is in fact logically
absurd. For we can readily describe conditions which, if realized, would
verify the sentence "The outside of the Chrysler Building is painted a bright
yellow"; but similarly, we can describe verifying conditions for its denial;
hence, according to the quoted principle, both the sentence and its denial
would have to be considered true. Incidentally, the passage under discussion
does not accord with Werkmeister's perfectly correct observation, l. c., p. 40,
that verifiability is intended to characterize the meaning of a sentence--which
shows that verifiability is meant to be a criterion of cognitive significance
rather than of truth.
[7] The arguments here adduced against the verifiability
criterion also prove the inadequacy of a view closely related to it, namely
that two sentences have the same cognitive significance if any set of
observation sentences which would verify one of them would also verify the
other, and conversely. Thus, e. g., under this criterion, any two general laws
would have to be assigned the same cognitive significance, for no general law
is verified by any set of observation sentences. The view just referred to
must be clearly distinguished from a position which Russell examines in his
critical discussion of the positivistic meaning criterion. It is "the theory
that two propositions whose verified consequences are identical have the same
significance" (RUSSELL, B., Human Knowledge, Simon and Schuster, New York, 1948, p. 448). This view is untenable indeed, for what
consequences of a statement have actually been verified at a given time is
obviously a matter of historical accident which cannot possibly serve to
establish identity of cognitive significance. But I am not aware that any
logical positivist ever subscribed to that "theory".
[8] The idea of using theoretical falsifiability by
observational evidence as the "criterion of demarcation" separating empirical
science from mathematics and logic on the one hand and from metaphysics on the
other is due to K. Popper (cf. POPPER, K., Logik der Forschung, Springer, Wien, 1935.
[Subsequently translated as The Logic of Scientific Discovery (revised)
Harper Torchbook: 1965 in paperback.], section 1-7 and 19-24; also see POPPER, K., The Open Society and its Enemies, 2 vols., Routledge,
London, 1945, vol.
11, pp., 282-285). Whether Popper would subscribe to the proposed restatement
of the falsifiability criterion, I do not know.
[9] AYER, A. J., Language, Truth and Logic, Oxford Univ. Press, 1936;
2nd ed., Gollancz, London, 1946., Ch. I.--The case against the requirements of
verifiability and of falsifiability, and favor of a requirement of partial
confirmability and disconfirmability is very clearly presented also by Pap in
PAP, A., Elements of Analytic Philosophy, The Macmillan Co., New
York, 1949, Chapter 13.
[10] AYER, A. J., Language, Truth and Logic, Oxford Univ. Press, 1936;
2nd ed., Gollancz, London, 1946, 2d ed., pp. 11-12.
[11] According to Stace (cf. STACE, W. T., Positivism (Mind, vol. 53, 1944), p. 218), the criterion of
partial and indirect testability, which he calls the positivist principle,
presupposes (and thus logically entails) another principle, which he terms the
Principle of Observable Kinds: "A sentence, in order to be significant, must
assert or deny facts which are of a kind or class such that it is logically
possible directly to observe some facts which are instances of that class or
kind. And if a sentence purports to assert or deny facts which are of a class
or kind such that it would be logically impossible directly to observe any
instance of that class or kind, then the sentence is non-significant." I think
the argument Stace offers to prove that this principle is entailed by the
requirement of testability is inconclusive (mainly because of the incorrect
tacit assumption that "on the transformation view of deduction", the premises
of a valid deductive argument must be necessary conditions for the conclusion
(l. c., p. 225)). Without pressing this point any further, I should like to
add here a remark on the principle of observable kinds itself. Professor Stace
does not say how we are to determine what "facts" a given sentence asserts or
denies, or indeed whether it asserts or denies any "facts" at all. Hence, the
exact import of the principle remains unclear. No matter, however, how one
might choose the criteria for the factual reference of sentences, this much
seems certain: If a sentence expresses any fact at all, say f, then it
satisfies the requirement laid down in the first sentence of the principle; for
we can always form a class containing f together with the fact expressed
by some observation sentence of our choice, which makes f a member of a class
of facts at least one of which is capable, in principle, of direct observation.
The first part of the principle of observable kinds is therefore all-inclusive,
somewhat like Ayer's original formulation of the empiricist meaning
criterion.
[12] This restriction is expressed in recursive form and
involves no vicious circle. For the full statement of Ayer's criterion, see AYER, A. J., Language, Truth and Logic, Oxford Univ. Press, 1936;
2nd ed., Gollancz, London, 1946. [Subsequently in Dover paperback.]
, 2d edition, p. 13.
[13] Church, A., Review of AYER, A. J., Language, Truth and Logic, Oxford Univ. Press, 1936;
2nd ed., Gollancz, London, 1946. [Subsequently in Dover paperback.], 2nd. ed. (The Journal of Symb. Logic,
vol. 14, 1941), pp. 52-53).
[14] Important contributions to the problem have been made
by Carnap, R., Der logische Aufbau der Welt, Berlin, 1928. [Subsequently translated
by Rolf A. George as The Logical Structure of the World (Univ. of
California Press: 1967); paperback in 1969.] and by Goodman, N., The Structure of Appearance, To be published soon,
probably by Harvard University Press. [Harvard: 1951; second edition,
Bobbs-Merrill: 1966 in paperback.].
[15] This difficulty in the definition of disposition terms
was first pointed out and analyzed by Carnap (in CARNAP, R., Testability and meaning (Philos. of Science, vol.
3, 1936, and vol. 4, 1937); see esp. section 7).
[16] The concept of strict implication as introduced by C.
I. Lewis would be of no avail for the interpretation of the strong "if...
then..." as here understood, for it refers to a purely logical relationship of
entailment, whereas the concept under consideration will, in general, represent
a nonomological relationship, i. e., one based on empirical laws. For recent
discussions of the problems of counterfactuals and laws, see Langford, C. H., Review in The Journal of Symb. Logic, vol. 6
(1941), pp. 67-68.;
Lewis, C. I., An Analysis of Knowledge and Valuation, Open Court
Publ., La Salle, Ill., 1946, pp. 210-230; Chisholm, R. M., The contrary-to-fact conditional (Mind,
vol. 55, 1946); Goodman, N., The problem of counterfactual conditionals (Journal
of Philos., vol. 44, 1947); Reichenbach, H., Elements of Symbolic Logic, The Macmillan Co., New York,
1947, Chapter
VIII; Hempel, C.G. and Oppenheim, P., Studies in the logic of
explanation (Philos. of Science, vol. 15, 1948). [Subsequently reprinted in Feigl, H.,
& Brodbeck, M., (eds.), Readings in the Philosophy of Science
(Appleton-Century-Crofts, Inc.: 1953.] [p. 63:] , Part 111; Popper, K., A note on natural laws and so-called "contrary-to-fact
conditionals" (Mind, vol. 58, 1949).
[17] Cf. Carnap, R., Testability and meaning (Philos. of Science, vol.
3, 1936, and vol. 4, 1937); a brief elementary exposition of the
central idea may be found in Carnap, R., Logical foundations of the unity of science, In:
Internat. Encyclopedia of Unified Science, 1, 1; Univ. of Chicago Press,
1938, Part III. The partial definition (R)
formulated above for the expression "T(x)= c" illustrates only the simplest
type of reduction sentence, the so-called bilateral reduction sentence.
[18] On the concept of operational definition, which was
developed by Bridgman, see, for example, Bridgman, P. W., The Logic of Modern Physics, The Macmillan Co., New York, 927, BRIDGMAN, P. W., Operational analysis (Philos. of Science,
vol. 5, 1938), and Feigl, H., Operationism and scientific method (Psychol.
Review, vol. 52, 1945). (Also reprinted in Feigl and Sellars, Readings in
Philosophical Analysis, New York, 1949.).
[19] The distinction between a formal deductive system and
the empirical theory resulting from it by an interpretation has been elaborated
in detail by Reichenbach in his penetrating studies of the relations between
pure and physical geometry; cf., e. g., Reichenbach, H., Philosophic der Raum-Zeit-Lehre, Berlin, 1928. The method by means
of which a formal system is given empirical content is characterized by
Reichenbach as "coordinating definition" of the primitives in the theory by
means of specific empirical concepts. As is suggested by our discussion of
reduction and the interpretation of theoretical constructs, however, the
process in question may have to be construed as a partial interpretation of the
non-logical terms of the system rather than as a complete definition of the
latter in terms of the concepts of a thing-language.
[20] These systems have not been characterized here as fully
and as precisely as would be desirable. Indeed, the exact character of the
empirical interpretation of theoretical constructs and of the theories in which
they function is in need of further investigation. Some problems which arise
in this connection--such as whether, or in what sense, theoretical constructs
may be said to denote--are obviously also of considerable epistemological
interest. Some suggestions as to the interpretation of theoretical constructs
may be found in Carnap, R., Foundations of logic and mathematics. Internal.
Encyclopedia of Unified Science, 1, 3; Univ. of Chicago Press, 1939, section 24. and in Kaplan, A., Definition and specification of meaning (Journal of Philos.,
vol. 43, 1946); for an excellent
discussion of the epistemological aspects of the problem, see Feigl, H., "Existential hypotheses; realistic vs. interpretations,"
(Philos. of Science, vol. 17, 1950)..
[21] This point is clearly taken into consideration in
Ayer's criteria of cognitive significance, which were discussed in section 2.
[22] For a fuller discussion of the Issues here involved cf.
Feigl, H., "Existential hypotheses; realistic vs. interpretations,"
(Philos. of Science, vol. 17, 1950), and the comments on Feigl's position which will be published
together with that article.
[23] In the preface to the second edition of his book, Ayer
takes a very similar position: be holds that the testability criterion is a
definition which, however, is not entirely arbitrary, because a sentence which
did not satisfy the criterion "would not be capable of being understood in the
sense in which either scientific hypotheses or commonsense statements are
habitually Understood" (AYER, A. J., Language, Truth and Logic, Oxford Univ. Press, 1936;
2nd ed., Gollancz, London, 1946., p. 16).
[24] Cf. Carnap's characterization of explication in his
article, CARNAP, R., The two concepts of probability (Philos. and Phenom.
Research, vol. 5, 1945), which examines in outline the explication of the concept of
probability. [Boardman's note: See Carnap's discussion of "explication" from his Logical Foundations of Probability (1950).] The Frege-Russell definition of integers as classes of equivalent
classes, and the semantical definition of truth--cf. Tarski, A., The semantic conception of truth and the foundations of
semantics (Philos. and Phenom. Research, vol. 4, 1944) (Also reprinted in
Feigl and Sellars, Readings in Philosophical Analysis, New York, 1949.)--are
outstanding examples of explication. For a lucid discussion of various aspects
of logical analysis see Pap, A., Elements of Analytic Philosophy, The Macmillan Co., New
York, 1949, Chapter 17.
[25] Thus, e. g., our criterion qualifies as significant
certain statements containing, say, thousands of existential or universal
quantifiers-even though such sentences may never occur in every-day nor perhaps
even in scientific discourse. For indeed, from a systematic point of view it
would be arbitrary and unjustifiable to limit the class of significant
statements to those containing no more than some fixed number of quantifiers.
For further discussion of this point, cf. Carnap, R., Testability and meaning (Philos. of Science, vol.
3, 1936, and vol. 4, 1937), sections 17, 24, 25.
Bibliographic References
(1) AYER, A. J., Language, Truth and Logic, Oxford Univ. Press, 1936;
2nd ed., Gollancz, London, 1946. [Subsequently in Dover paperback.]
(2) BENJAMIN, A. C., Is empiricism self-refuting? (Journal of
Philos., vol. 38, 1941).
(3) BRIDGMAN, P. W., The Logic of Modern Physics, The Macmillan Co., New
York, 927.
(4) BRIDGMAN, P. W., Operational analysis (Philos. of Science,
vol. 5, 1938).
(5) CARNAP, R., Der logische Aufbau der Welt, Berlin, 1928. [Subsequently translated
by Rolf A. George as The Logical Structure of the World (Univ. of
California Press: 1967); paperback in 1969.]
(6) CARNAP, R., Testability and meaning (Philos. of Science, vol.
3, 1936, and vol. 4, 1937).
(7) CARNAP, R., Logical foundations of the unity of science, In:
Internat. Encyclopedia of Unified Science, 1, 1; Univ. of Chicago Press,
1938.
(8) CARNAP, R., Foundations of logic and mathematics. Internal.
Encyclopedia of Unified Science, 1, 3; Univ. of Chicago Press, 1939.
(9) CARNAP, R., The two concepts of probability (Philos. and Phenom.
Research, vol. 5, 1945).
(10) CHISHOLM, R. M., The contrary-to-fact conditional (Mind,
vol. 55, 1946).
(11) CHURCH, A., Review of (1), 2nd. ed. (The Journal of Symb. Logic,
vol. 14, 1941), pp. 52-53).
(12) FEIGL, H., Operationism and scientific method (Psychol.
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Philosophical Analysis, New York, 1949.)
(13) FEIGL, H., Existential hypotheses; realistic vs. interpretations,
(Philos. of Science, vol. 17, 1950).
(14) GOODMAN, N., The problem of counterfactual conditionals (Journal
of Philos., vol. 44, 1947).
(15) GOODMAN, N., The Structure of Appearance, To be published soon,
probably by Harvard University Press. [Harvard: 1951; second edition,
Bobbs-Merrill: 1966 in paperback.]
(16) HEMPEL, C. G., and Oppenheim, P., Studies in the logic of
explanation (Philos. of Science, vol. 15, 1948). [Subsequently reprinted in Feigl, H.,
& Brodbeck, M., (eds.), Readings in the Philosophy of Science
(Appleton-Century-Crofts, Inc.: 1953.] [p. 63:]
(17) KAPLAN, A., Definition and specification of meaning (Journal of Philos.,
vol. 43, 1946).
(18) LANGFORD, C. H., Review in The Journal of Symb. Logic, vol. 6
(1941), pp. 67-68.
(19) LECOMTE Du NOÜY, Human Destiny, New York, London, Toronto,
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(20) LEWIS, C. I., An Analysis of Knowledge and Valuation, Open Court
Publ., La Salle, Ill., 1946.
(21) PAP, A., Elements of Analytic Philosophy, The Macmillan Co., New
York, 1949.
(22) POPPER, K., Logik der Forschung, Springer, Wien, 1935.
{Subsequently translated as The Logic of Scientific Discovery (revised)
Harper Torchbook: 1965 in paperback.]
(23) POPPER, K., The Open Society and its Enemies, 2 vols., Routledge,
London, 1945.
(24) POPPER, K., A note on natural laws and so-called "contrary-to-fact
conditionals" (Mind, vol. 58, 1949).
(25) REICHENBACH, H., Philosophic der Raum-Zeit-Lehre, Berlin, 1928.
(26) REICHENBACH, H., Elements of Symbolic Logic, The Macmillan Co., New York,
1947.
(27) RUSSELL, B., Human Knowledge, Simon and Schuster, New York, 1948.
(28) SCHLICK, M., Meaning and Verification (Philos. Review, vol.
45,1936). (Also reprinted in Feigl and Sellars, Readings in Philosophical
Analysis, New York, 1949).
(29) STACE, W. T., Positivism (Mind, vol. 53, 1944).
(30) TARSKI, A., The semantic conception of truth and the foundations of
semantics (Philos. and Phenom. Research, vol. 4, 1944) (Also reprinted in
Feigl and Sellars, Readings in Philosophical Analysis, New York, 1949.)
(31) WERKMEISTER, W. H., The Basis and Structure of Knowledge, Harper,
New York and London, 1948.
(32) WHITEHEAD, A. N., and RUSSELL, B., Principia Mathematica, 3 vols.,
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