University of Virginia Library

POSITIVISM IN THE
TWENTIETH CENTURY
(LOGICAL EMPIRICISM)

The basic ideas of this movement in twentieth-century
philosophy of science, had their historical roots in the
philosophies of the Enlightenment and more generally
in classical British empiricism (particularly in David
Hume) as well as in nineteenth-century positivism
(notably Auguste Comte, John Stuart Mill, and Richard
Avenarius). Equally, if not even more important were
the influences that came from outstanding scientist-
philosophers of the same century, e.g., G. F. B.
Riemann, H. von Helmholtz, Ernst Mach, Heinrich
Hertz, Ludwig Boltzmann, Henri Poincaré, David
Hilbert—and in the early twentieth century, especially
Albert Einstein; the incisive impact of the great math-
ematical logicians, primarily Gottlob Frege, Bertrand
Russell (also through his theory of knowledge), Alfred
North Whitehead, together with all the aforemen-
tioned influences resulted in the 1920's in the formation
of the Vienna Circle of Logical Positivsts and The
Berlin Society of Scientific Empiricists. The Vienna
group was formed in 1924 under the leadership of
Moritz Schlick. Its most active members were Hans
Hahn (a mathematician, and an admirer of Russell's),
Kurt Reidemeister (also a mathematician who called
the circle's attention to Ludwig Wittgenstein's Tracta-
tus Logico-Philosophicus), Otto Neurath (sociolo-
gist-economist and an energetic organizer and propa-
gandist of the Circle), Friedrich Waismann; and, after
1926, especially Rudolf Carnap with his important
work in modern logic, the foundations of mathematics,
concept-formation in physics, and a logical systemati-
zation of the concepts of empirical knowledge in gen-
eral (cf. his The Logical Structure of the World, original
German edition, 1928). Schlick had paved the way for
much that became the standpoint of logical positivism
in his Allgemeine Erkenntnislehre (“General Theory of
Knowledge”), first edition as early as 1918; second
edition 1925. Wittgenstein, though admired by most
members of the Vienna Circle, and despite frequent
conversations with some circle members (especially
Schlick, Waismann, Carnap, Feigl) never joined or even
attended the meetings of the Circle. Among the
younger members were the mathematicians Kurt Gödel
and Karl Menger, and later Gustav Bergmann. Also,
despite basic similarities along the lines of philo-
sophical endeavor, there were Edgar Zilsel and Karl
R. Popper who, probably because of some divergencies
they considered very essential, did not become
members—but were close friends with some of them.
Victor Kraft was a member from the beginning, but

some of his most important published work appeared
long after the final disintegration of the Circle. On
the whole the group consisted mainly of scientifically
or mathematically oriented philosophers, or of philo-
sophically gifted scientists or mathematicians.

The Berlin group was led by Hans Reichenbach
whose early training and experience had been, like
Carnap's, in modern physics and mathematics. Richard
von Mises (a great mathematician, aerodynamicist, and
positivist philosopher—very much like his friend
Philipp Frank, the Prague theoretical physicist who
like von Mises was Viennese by origin and an out-
standing disciple of Ernst Mach's) along with Kurt
Grelling, Walter Dubislav, and the younger student-
members C. G. Hempel and Olaf Helmer were among
the principal exponents of the Berlin group.

In highly compressed and somewhat oversimplified
form the main ideas (common to the Vienna and the
Berlin groups) comprised:

(1) A view—considered then new and revolutionary
by its proponents—of the “true” nature and “genuine”
task of the philosophical enterprise. In contradis-
tinction to the still largely prevailing speculative (e.g.,
Hegelian) tendencies in (transcendent) metaphysics, the
Viennese and Berliners were convinced that most (if
not all) allegedly unsolvable problems of philosophy
(the unanswerable “riddles of the universe” regarding
which even some nineteenth-century scientists like E.
DuBois-Reymond declared a stern ignoramus et ig-
norabimus) rest on conceptual confusions or on closely
related misuses of language. It was especially under
Wittgenstein's influence that the primary (if not the
sole) task of a sound philosophy was considered as a
kind of “therapy” of thought. Inspired by this veritable
bouleversement, H. Feigl impudently defined philoso-
phy as “the disease of which it should be the cure.”
(This may have been an unwitting plagiarism or para-
phrase of the witticism of Vienna's great political
satirist, Karl Kraus, who had said that “psychoanalysis
is the disease whose therapy it pretends to be”). Re-
flection upon the very logic of explanation (be it
commonsensical or scientific) showed clearly that, for
example, the still fashionable existentialist questions:
“Why is there anything at all?” or “Why is what there
is the way it is?” are unanswerable not because they
are too difficult, or surpass the limits of human intelli-
gence altogether, but because all (legitimate) explana-
tion (in contradistinction to tranquilization by means
of verbal sedatives) inevitably proceeds from premisses
which are themselves unexplained, at least in the given
context of inquiry. Moreover, explanations of facts (or
events) or of the regularities (of facts or events) require
premisses in the form of (deterministic, or else proba-
bilistic) laws—and these laws depend in their validity
on confirming evidence. And since confirming (or dis-
confirming) evidence is apt to change—often in rather
surprising ways—knowledge-claims (most especially in
“higher level” scientific hypotheses or theories) can be
held only tentatively (“until further notice”), i.e., they
must be kept open for revision. The only “safe” proce-
dure is therefore that of the critical approach or the
open mind. Genuine and legitimate explanation of
facts, events (and their regularities) is thus relative in
two ways: (a) only in the light of tentatively assumed
premisses can we say the facts are “necessarily so” (i.e.,
are deductively derivable from those premisses); (b) the
premisses themselves stand or fall in the light of obser-
vational evidence.

Pseudoproblems in the history of thought (even of
scientific thought) have often arisen out of (unwittingly)
making some assertions “proof against disproof,” i.e.,
by making completely and essentially untestable as-
sumptions. Many a scientific hypothesis originated
as a testable, that is, confirmable or disconfirmable
knowledge-claim; but when difficulties arose, often by
a shift in meaning, those hypotheses were rendered,
in principle, impervious to any conceivable test. Well-
known important examples are the doctrines of abso-
lute space and time (Plato, Newton); of substance
(Locke); of “necessity” in causality (already incisively
and classically criticized by Hume); of the vital force
(“entelechies,” etc.) assumed by many vitalists as an
explanation of the admittedly most puzzling features
of organic life; the ether hypothesis in its last desperate
stand (Lorentz, Fitzgerald); and so forth. Great scien-
tific innovators (like Lavoisier in the case of the phlo-
giston theory in chemistry; or Einstein in the case of
the ether, and of space and time generally) recognized
the spurious nature of such explanations and replaced
them by scientifically legitimate ones. According to the
Vienna and Berlin positivists and empiricists, the re-
form of philosophy was to be patterned after the
paradigm of those great purifications and clarifications
in the sciences.

(2) Perhaps the most distinctive feature of the new
empiricism was the pivotal role of its analysis of lan-
guage and meaning. None of the Europeans were
aware (in the middle 1920's) of the important work
of Charles S. Peirce, the great American philosopher
and logician who had anticipated (in 1878) in an in-
formal way the basic idea of the notorious “verifiability
criterion” of meaning. Nor was there—at the time—
much awareness of the pragmatism of William James
(who was strongly influenced by Peirce's ideas). The
Viennese, at least in the manner in which they con-
strued the often obscurely aphoristic Tractatus Logico-
Philosophicus of Ludwig Wittgenstein, were emphatic
in declaring testability-in-principle a necessary condi-

tion for the factual meaningfulness of sentences. The
sentences in a language (be it the language of common
usage, of science, or of metaphysics) were said to make
factual sense only if it was logically conceivable that
they might be confirmed or disconfirmed (i.e., at least
partially and/or indirectly verified or refuted) by
empirical evidence. From this point of view (more fully
developed in the thirties by the American philosopher
Charles Morris, but already contained in the slightly
earlier theory of language developed by the great
Vienna psychologist Karl Bühler) fundamental distinc-
tions among the various functions of language (or com-
munication quite generally) and the corresponding
types of significance were made. Preoccupied with
science, the focus of interest was centered upon the
informative or representative function of language (and
the cognitive meanings). This was distinguished from
the expression and appeal functions, i.e., mainly the
pictorial, the emotional, and the motivative uses of
communication. It was granted (even emphasized) that
in most cases these various functions of language are
fused or combined; and the distinctions made were the
result of a logico-philosophical analysis. Thus while
granting the fusion, the logical positivists warned
against confusion of one type of significance with
another. The sort of metaphysics that was repudiated
was said to arise out of mistaking noncognitive (picto-
rial, emotional, motivative) significance for genuinely
cognitive (representative) meaning. Among the
genuinely cognitive meanings a further very important
distinction was drawn between the purely formal (i.e.,
logico-mathematical) and the factual (empirical) types
of meaning. Full awareness of this distinction led,
among other things, to a repudiation of the “conven-
tionalist” doctrine (suggested by H. Poincaré and car-
ried to an untenable exaggeration by Hugo Dingler)
according to which the principles of physical geometry
and the basic “laws of nature” generally, were consid-
ered as “definitions in disguise.” The logical empiricist
doctrine (later attacked as one of its “dogmas” by the
prominent American logician Willard Van Orman
Quine) insisted on the indispensable distinction (already
contained in Hume's Treatise, and explicitly—but
rather narrowly drawn in Kant's Critique of Pure Rea-
son) between analytic and synthetic propositions.
Analytic propositions are true by virtue of presupposed
meanings (which can be articulated by explicit defini-
tions or meaning rules), whereas synthetic propositions
are nonanalytic, and thus require grounds of validity
outside of mere meaning assignments or definitions.
The logical positivists—being staunch empiricists—
recognized only the data of experience as the grounds
of validity for synthetic knowledge-claims. “Pure rea-
son” was considered competent only in the realm of
analytic truth. “Pure intuition” (in Kant's or any other
sense) was at best admitted as a source of ideas
(“hunches”) but never as a basis of validity.

In the further development (during the 1930's) of
the analyses of language, and mainly in the work of
Carnap, it became clear that the language which is
the object of discussion (the “object language”) must
be distinguished from an (especially constructed)
metalanguage which talks about the object-language.
Moreover, purely syntactical studies regarding the
formation and transformation rules (i.e., the purely
“structural” aspects of language) soon required (as
Alfred Tarski's important work indicated) supple-
mentation by semantical analyses (concerned with rules
of designation and of truth). The purely syntactical
approach was prevalent in the formalist philosophy of
mathematics of Hilbert, Bernays, and their disciples.
The semantical approach was actually implicit in the
work of Frege, Russell, Whitehead, and was made fully
explicit by Tarski and Carnap.

The theories of the empirical sciences especially
those of physics, were viewed as consisting of erstwhile
uninterpreted postulates, containing basic (undefined,
or rather only “implicitly” defined) “primitive” con-
cepts; explicit definitions which introduce more com-
plex concepts; and correspondence rules (or, as
Reichenbach called them, “coordinative definitions”)
which provided at least a partial empirical inter-
pretation of the “primitive” concepts. The corre-
spondence rules connect the abstract concepts of the
postulates with the empirical (experimental, mensura-
tional) concepts of the observation language. Thus the
factual-empirical significance “seeps upward” to the
originally quite abstract (“formal”) concepts of the
postulate system.

(3) A characteristic feature, especially of Viennese
positivism in epistemology and philosophy of science,
was two doctrines or “theses” of reductionism. The
first concerned the reducibility of the concepts of the
factual sciences to the concepts of a common observa-
tion basis. This view may be considered as a logical
version of the older empiricist (e.g., Humean) doctrine
of the relation of ideas to impressions. Influenced by
the rather sketchy attempts in this direction by Mach,
Avenarius, and the early Russell, Carnap in his early
Der Logische Aufbau der Welt (now also available in
English translation, The Logical Structure of the World)
presented a systematic rational reconstruction of the
major domains of empirical knowledge in terms of
concepts introduced by stepwise definitions with a
“ground level” of concepts pertaining to the data of
direct experience. This was essentially a reconstruction
along the lines of an epistemological phenomenalism.
A few years later Carnap came to prefer a different

basis for reconstruction (already briefly discussed as one
of several possible alternatives in the Aufbau): the
language that would provide for the “unity of science”
in this sense was to be the (“physicalistic”) intersubjec-
tive observation language. Once this starting point of
reconstruction was chosen, the unwelcome associations
of the earlier reconstruction with subjective idealism
or “methodological solipsism” were obviated. In less
technical terms it may be said that this first thesis of
physicalism—or of the unity of the language of science
simply amounted to asserting common, communicable
perceptual experience to be the ultimate testing
ground for all sorts of factual knowledge claims. With
the help of symbolic logic (essentially Whitehead-
Russell's) Carnap elaborated an impressive system of
definitions—including some completely worked out
examples—that was to show that the concepts of
empirical knowledge (especially those of the natural
and social sciences, including psychology) were thus
reducible to a minimum (in Carnap's system actually
a single) basic concept of immediate experience. While
the Aufbau was regarded by many of its critics
a—however brilliant—tour de force, it was patterned
after the exemplary sort of reduction presented by
Whitehead and Russell in their famous Principia Math-
ematica (3 vols., 1910-13; 1925). There the bold claim
was made that all of mathematics (really only set the-
ory, arithmetic, number theory, analysis) could be built
up from (or reduced to) a few principles and concepts
of modern logic. (Frege had done most of the important
spade work in that direction.) If Carnap's analogical
attempt for the empirical sciences succeeded, it would
represent a formal justification of the phenomenalist
epistemology according to which all factual knowl-
edge-claims are re-translatable into statements about
actual or possible immediate experience. Carnap, and
along with him most of the Viennese positivists
(Schlick, then under Carnap's and Wittgenstein's influ-
ence, Hahn, Frank, et al.) regarded the issue of realism
vs. phenomenalism as a pseudoproblem. Reichenbach,
however, insisted on an inductive critical realism ac-
cording to which the inference to an external world
(as well as other persons' mental states) was justifiable
on grounds of analogy and probability. This contro-
versy (by staunch positivists regarded as a dispute about
the “emperor's clothes”) still continues, e.g., in the issue
regarding realistic vs. instrumentalistic interpretations
of scientific theories. According to the realist view-
point, “existential hypotheses” (i.e., assumptions con-
cerning the reality of unobserved and even unobserv-
able entities such as atoms, electrons, and the host of
other subatomic particles; the unconscious wishes, mo-
tives, etc. as formulated in psychoanalytic theories)
were to be taken as referring to actually existing
“things” or occurring processes. In sharp opposition,
the instrumentalists viewed the concepts of such unob-
servable entities either as in principle dispensable logi-
cal constructions or, in accordance with earlier posi-
tivist tendencies, as (at best!) useful auxiliary fictions.

Retrospectively, the first thesis of the unity of science
(as championed by Carnap, Neurath, and Hahn, ca.
1928-35) seems relatively obvious (insofar as it is cor-
rect), and hardly worth the excitement and opposition
it aroused at the time. That thesis asserted neither a
unity of method nor a unity of explanatory premisses
for all of science. As already indicated, it merely in-
sisted on a common empirical basis or testing ground.

(4) The second thesis of physicalism or of the unity
of science was proposed by Carnap more in the sense
of a promising research program than as a truth about
science and its relation to the world, let alone as an
accomplished achievement along the lines of a unifica-
tion of the sciences. It did assert the logical possibility
and the empirical plausibility of a unitary set of ex-
planatory assumptions from which the empirical laws
and (with the help of descriptions of “initial and
boundary conditions”) even all the individual facts and
events of the world could (in principle) be derived.
This was, of course, a vast and precarious extrapolation
from whatever successes had been scored in the reduc-
tion (now in the sense of explanation) of empirical laws
to unifying theories; and of theories of lower level to
higher level theories. Outstanding examples were the
nineteenth-century reduction of optics to electromag-
netics; of part of thermodynamics to the kinetic theory
of heat (developed in molecular and statistical me-
chanics); and the strong indications of the reducibility
of the laws of chemistry (and the nature of the chemical
bond) to atomic theory, implemented especially by the
new quantum and wave mechanics (beginning in 1926).

The meaning of this (second) thesis of the unity of
science is best understood by considering what it tries
to exclude or oppose, viz., the doctrines of Emergent
Evolution, and to some extent also the related views
of holism. These philosophies of science (or of nature)
insist on some absolute irreducibilities, especially that
of biology and psychology (and a fortiori of the social
sciences) to basic physics. Representatives of these
ideologies oppose the “reductionism” of physicalism
(or of the second unity of science thesis). They maintain
that in the course of evolution entirely new levels of re-
ality emerged, whose regularities are autonomous, i.e.,
insusceptible to explanation on the basis of the theories
and laws that seem to suffice for the phenomena of
the inorganic world. It is important to note that
emergentism need not be combined let alone buttressed
with vitalism. The assumption of an extra-physical vis
vitalis (vital force, entelechy, élan vital, etc.), as

formulated in most forms (old or new) of vitalism, is
usually without any genuine explanatory power. It has
served altogether too often as an intellectual tran-
quilizer or verbal sedative—stifling scientific inquiry
rather than encouraging it to proceed in new direc-
tions.

Emergentist doctrines, though by no means easy to
formulate clearly and positively, need not be trans-
empirical. Just as electromagnetics turned out to be
irreducible to classical Newtonian mechanics, so biol-
ogy, despite the remarkable advances of biophysics and
biochemistry (and more recently especially of molecu-
lar biology), may ultimately be “emergent” in relation
to basic physics. In any case the recognition of the
all-important role of organization, structure, configu-
ration stressed at first by the Gestalt psychologists
(especially M. Wertheimer and W. Köhler) is entirely
compatible with a nonmechanistic physicalism. To be
sure, some of the positivists (notably G. Bergmann)
opposed any and all holistic tendencies, staunchly
maintaining that organic wholes could be analyzed as
composed of parts, and their different features could
be explained by adducing composition laws (such as
those expressible in vector algebra, e.g., in the simplest
case: the parallelogram of forces). But while this is
entirely appropriate within the theoretical schemes of
classical physics (from Newton to Einstein), it fits into
neither the conceptual framework of electrodynamic
field theory nor (especially) that of quantum physics
(with its principle of complementarity and Pauli's
exclusion principle—a sort of “Gestalt” law on the
atomic level).

Hence, even despite the astonishing and dramatic
developments in molecular biology (the double helix
model of the gene; the DNA and RNA stories, etc.)
the thesis of a unitary explanation for all phenomena
of nature is still at best a “promissory note,” i.e., a
bold conjecture regarding the future development of
the sciences. The logical empiricists have always con-
sidered that thesis as a general research program that
helps in encouraging reduction without being reduc-
tionistic in a dogmatic sense.

The difficulties multiply rapidly in regard to psy-
chology and the social sciences. The positivists, along
with Bertrand Russell (and some pragmatists like Edgar
A. Singer), for a while joined the bandwagon of Ameri-
can behaviorists (J. B. Watson, A. P. Weiss, C. L. Hull,
B. F. Skinner, et al.). Actually, Carnap, as early as 1932,
formulated in fairly detailed outline a kind of logical
behaviorism which only later (and quite independently)
was quite elaborately expounded and defended in
Gilbert Ryle's Concept of Mind (1949), and in a manner
designed primarily for psychologists by B. F. Skinner
in Science and Human Behavior (1953). The basic
intention in Skinner's approach was to show that psy-
chology could be presented as a branch of natural
science. Ryle's work (no doubt influenced by Wittgen-
stein—at least as Ryle understood the later work of
Wittgenstein) was designed to show that the very
“grammar” of our common language requires an inter-
subjective approach even in regard to such notoriously
“private” mental acts as thought, imagery, emotion,
etc. Carnap's ideas on the scientific status of psycho-
logical knowledge-claims are to be taken as an outcome
primarily of the first thesis of the unity of science. Since
the testing-basis of psychology if it is to be scientific
must be in the data of everyday or experimental obser-
vations of behavior (including verbal behavior), the
concepts and propositions of the science of mind must
be “reducible” (in the sense of the first thesis) to the
concepts regarding the overt behavior of organisms
(man included, of course). In the spirit of the second
thesis Carnap argued that conceivably (in the more
or less distant future) the facts and regularities of mind
might well become explainable, and thus “reduced”
in the other sense, to neurophysiology, and, if the
second thesis should prove correct, eventually to basic
physics.

Philosophical opposition naturally arose on this last
claim. This despite the fact that Carnap always stressed
the conjectural character of the second (“unitary”)
thesis. The traditional mind-body problems thus came
to the fore again, having been almost completely
suppressed during the reign of (first) phenomenalistic
and (later) behavioristic-physicalistic trends of thought.
Wittgenstein's arguments against the possibility of a
private language together with the prevailing Ameri-
can climate of psychological and philosophical views
of “mind” seemed, for quite a few years, to exclude
any revival of the well-known controversies of dualism
(interactionistic, emergentistic, or parallelistic) with
monism (materialistic, mentalistic, neutral, or various
forms of double aspect, double-language, twofold
knowledge identity theories). Carnap himself reluc-
tantly approved of some formulation of an identity
theory. But his preference remained for a view accord-
ing to which the mentalistic language of immediate
experience would be supplanted by a physicalistic
language. As a first step here (in approximate agree-
ment with G. Ryle) he tried to show how the many
dispositional concepts (designating abilities, capacities,
or propensities) could be construed by some sort of
conditional definitions (reduction sentences), i.e., by
test-situation causally implying test result conditionals.
But in an important later essay (1956), Carnap pre-
ferred to recognize most scientific concepts as theoret-
ical concepts, whose meaning is to be explicated by
postulates and correspondence rules. Others (like H.

Feigl) attempted to formulate an identity theory pat-
terned after the reductive identifications that are so
abundantly present in the natural sciences. Just as we
can in physics “identify” the intensity of heat of a
substance with the average kinetic energy of the mo-
lecular motions, so the mental acts and processes may
well be ultimately identified with some neurophysio-
logical processes occurring in the nervous systems (or
more specifically in the cerebral cortex) of the mam-
malian organisms. This “solution”—also formulated in
the recent work of J. J. C. Smart and other Australian
philosophers of science—is currently very much in the
focus of philosophical discussion and dispute.

Some of the more conspicuous difficulties of the new
“monism” have to do with the apparently irreducible
feature of intentionality that seems to characterize all
or most mental phenomena. The acts of thought, voli-
tion, perception, and such emotions as love, hatred,
hope, fear, etc., are all directed upon some object or
other be it an actually existing or a merely imagined
or fancied object. It seems well nigh impossible to find
in the neurophysiological (and a fortiori physical)
processes a structurally similar (isomorphic) counter-
part that would suggest an ultimate identity. The only
helpful way out was proposed by Wilfrid Sellars who
views the intentionality feature as basically linguistic.
If this is correct, the act-object relation, though
phenomenologically undeniable, is to be analyzed as a
case of the semantic relation of designation. If this is
correct, then we are here not dealing with the relation
of the mental to the physical, but rather with the
relation of the logical (semantical) to either the psy-
chological processes, or the physical processes in the
brain (or in the electronic computer). Since the logical
positivists always agreed with the rejection of psy-
chologism (i.e., the fallacious identification of logical
with psychological categories) already forcefully pre-
sented by Frege and Husserl, they did not feel that
the discussion of intentionality belonged in the domain
of the mind-body problem. But in recent discussions,
just this is being disputed again.

(5) The most important positive achievements of the
logical empiricists are contained in their work in phi-
losophy of the sciences. These cannot be briefly sum-
marized. Hence a few hints and the (appended) notes
and references must suffice. Carnap's work in syntax
and semantics has already been mentioned. Equally
important are his studies and those of Hans Reichen-
bach, Ernest Nagel, C. G. Hempel, and a host of
younger scholars (notably, Adolf Grünbaum, Wesley
Salmon, Mario Bunge, Wolfgang Stegmüller, and
others) in the logic and methodology of the empirical
sciences. Problems of induction and probability, of
space and time, of the interpretations of modern
physics (especially of the theory of relativity and of
quantum mechanics), in the philosophy of biology,
(J. H. Woodger, and more recently Morton Beckner and
Kenneth Schaffner), and in the logic of modern psy-
chology (C. C. Pratt, S. S. Stevens, H. Feigl, P. E.
Meehl, and others)—all these and many other contri-
butions have been growing enormously and have, on
the whole, been received with the greatest interest—
especially by many scientists in various fields, and by
scientifically informed and interested philosophers.
Several collaborative groups for research in the philos-
ophy of science have been formed, beginning with the
Minnesota Center (1953); and later the Departments
for Philosophy and History of Science at Indiana and
Pittsburgh Universities; the Boston Colloquium; (for a
few years also the Delaware Seminar). In most other
universities there are outstanding younger scholars
pursuing research and engaged in teaching philosophy
of science, all of them at least influenced by, or reacting
critically to the ideas of the logical empiricists.

The change of designation from “logical positivism”
to “logical empiricism” (around 1935) was due to the
increasing influence of Reichenbach's and Popper's
scientific realism and thus the final abandonment of
the (Hume-Mach) phenomenalism or sensationalism.
Even the empiricism of the group has been under
attack—first by Karl Popper's incisive critique of
“inductivism,” later by the repudiation of the analytic-
synthetic distinction by W. V. O. Quine and Hilary
Putnam; and finally the critique of the empiricist views
of scientific theories and of scientific explanation by
Paul K. Feyerabend and Thomas S. Kuhn. The original
line of logical empiricists (with some exceptions) had
not sufficiently focussed their attention on the history
of science; this made some of their pronouncements
open to severe criticisms. The current controversies
regarding the level structure of scientific explanation,
and the meaning invariance (or variance) of basic
scientific concepts reflect some of those recent doubts
regarding the adequacy of the logical empiricist ap-
proach.

A different school of thought, largely of British origin
(represented by S. Toulmin, W. H. Watson, N. R.
Hanson, and others), originated under the influence of
Ludwig Wittgenstein's later work. Analyses of meaning
of this kind are oriented along the lines of making
explicit the rules according to which the words or
symbols of language (whether of common speech or
of the scientific terminologies) are used. This is some-
what akin to the much earlier operationalistic ap-
proach that was first explicitly formulated by the
Harvard physicist, P. W. Bridgman. In fact at the time
Bridgman independently conceived his ideas, the simi-
larities with the contemporaneous Viennese positivism
were quite striking. In the author's opinion, both
operationalism and logical positivism, along with the

earlier pragmatism, were a salutary antidote or pre-
ventive in regard to metaphysical speculation, but their
lesson has been absorbed and a considerable liberaliza-
tion has succeeded, especially in the philosophy of
science.

BIBLIOGRAPHY

Most of the original “classics” of logical positivism, logical
empiricism (and of the related analytic and linguistic phi-
losophy)—both books and articles—are listed in the ample
bibliography of A. J. Ayer, ed., Logical Positivism (New York,
1959). Among many other important essays, R. Carnap's
“Psychology in Physical Language” is contained in this
anthology.

Books, mainly in the area of the foundations of the sci-
ences (but also in philosophy of language, epistemology),
many by the leading logical empiricists, are listed in the
ample, and fairly up-to-date Bibliography and Index by
Herbert Feigl and Charles Morris, eds., International Ency-
clopedia of Unified Science, Vol. II, No. 10. Of major rele-
vance are the works of R. Carnap, O. Neurath, M. Schlick,
P. Frank, H. Reichenbach, E. Nagel, C. G. Hempel, R. von
Mises, Charles Morris; and for criticisms, those of Karl R.
Popper; and the intellectual autobiography, the twenty-six
descriptive and critical essays, and Carnap's replies, in
P. A. Schilpp, ed., The Philosophy of Rudolf Carnap (LaSalle,
Ill., 1963).

For quite recent reactions, see P. Achinstein and S. F.
Barker, eds., The Legacy of Logical Positivism: Studies in
the Philosophy of Science (Baltimore, 1969).

For a brief account of the European movement of logical
positivism and its migration and impact in the Unites States,
see H. Feigl “The Wiener Kreis in America” in D. Fleming
and B. Baylin, eds., The Intellectual Migration: Europe and
America 1930-1960 (Cambridge, Mass., 1969). The early
history of Viennese positivism is well told in Victor Kraft's
The Vienna Circle, trans. A. Pap (New York, 1953); second
edition (somewhat expanded and revised) of The Wiener
Kreis (Vienna and New York, 1968). Another important
source is the monography by J. Joergensen, The Development
of Logical Empiricism, Vol. II, No. 9 of the International
Encyclopedia of Unified Science (Chicago, 1951).

HERBERT FEIGL

[See also Newton on Method; Positivism in Europe; Prag-
matism; Psychological Schools; Relativity; Unity of Sci-
encee to Kant.]