As indicated in Chapter 6, one of the main tenets of logical positivism was the verifiability principle. This principle can also be identified as the essential feature of science itself according to logical positivists. Recall that the verifiability principle holds that the meaning of a statement is to be found in its verification process. On the surface, to verify a claim means merely to demonstrate its truth. But this principle is about verifiability, not verification. “The meaning of a statement,” writes positivist Rudolf Carnap, “lies in the fact that it expresses a (conceivable, not necessarily existing) state of affairs” (1928/1967, p. 325). So the criterion is a hypothetical one: it is not about whether a specific claim is or is not verified but whether it is logically or theoretically possible to verify a specific claim. Again, according to Carnap, “One can know that a statement is meaningful even before one knows whether it is true or false” (1928/1967, p. 325). Thus, even a false claim could be considered scientific, in at least a formal sense. Taken broadly, the verifiability principle addresses the question of meaning in general, not purely within the domain and practice of science. In their discussions of meaning, the logical positivists commonly directed their criticisms and charges of meaninglessness against speculative metaphysical philosophers. Generally, of course, metaphysics is that branch of philosophy that studies reality and various concepts of reality. More specifically, what logical positivists mean by “metaphysics” is philosophy, which presumes “knowledge of a reality transcending the world of science and common sense” (Ayer, 1952/2000, p. 28), or “the field of alleged knowledge of the essence of things which transcends the realm of empirically founded, inductive science” (Carnap, 1932/1959, p. 80). The works of Martin Heidegger, Gottfried Hegel, and F. H. Bradley are often cited (and even mocked) as examples. Hegelian-influenced philosophers were especially prominent in Britain immediately before the development of logical positivism. Philosophers such as Bertrand Russell and G. E. Moore argued for a less idealist, more concrete, and empirical philosophy than what the neo-Hegelians practiced. Neo-Hegelians spoke in terms of conceptually vague notions like the “absolute.” Religious terms such as soul, spirit, and god would also fall into this category for the logical positivists. Based on this “knowledge” beyond science and common sense, metaphysicians would reach what A. J. Ayer characterized as “startling conclusions”: “time and space are unreal, or that nothing really moves, or that there are not many things in the Universe but only one, or that nothing which we perceive through our senses is real or wholly real, or that there is no such thing as matter, or no such things as minds” (Ayer, 1970, p. 64). It did not help that Hegel and Hegelians often repudiated logic as it is traditionally understood. One of the central concepts of Hegel’s philosophy, the dialectic, affirms the meaningfulness of contradictions.

The problem with these metaphysical terms is that they refer to proposed entities that would be “super-empirical” and unable to be deduced from empirical knowledge (Ayer, 1952/2000, p. 28). These metaphysicians often referred to some form of innate knowledge or rational intuition on which they based their knowledge of such entities. The Russell/Moore/Logical Positivist turn toward strict empiricism illustrated that these claims from rational intuition were without foundation, because no two metaphysicians’ rational intuitions seemed completely consistent. What one metaphysician might claim is supported by his rational intuition, another might deny. And there is no independent, objective standard against which to judge the disputed intuitive claims. An empirical standard for knowledge provides, so the logical positivists believed, an independent, objective standard for knowledge claims. By neglecting this empirical standard the metaphysician “produces sentences which fail to conform to the conditions under which alone a sentence can be literally significant” (Ayer, 1952/2000, p. 29). In other words, metaphysicians do not so much produce false claims as they produce meaningless or nonsensical claims.

What are these conditions under which a sentence can be significant? According to logical positivists, to be meaningful, a statement must be analytically true, self-contradictory, or empirically verifiable. These criteria imply the principle of the analytic/synthetic distinction that was an integral element in the logical positivist philosophy. According to this distinction, all statements (assertive sentences) are analytic, self-contradictory, or synthetic. An analytic statement is one in which the predicate is included in the subject. The classic example is “All bachelors are unmarried.” What makes this an analytic statement is that the concept of “being unmarried” is included in the concept of “bachelor.” Another simple example would be “The red ball is red.” Here again, the predicate “red” is included in the subject “red ball.” Given that the predicate of an analytic statement is included in its subject, another quality of analytic statements that follows from this is that they are necessarily true. This quality might also be referred to as being analytically true or true by definition. That is, no matter what, a true analytic statement will be true. Its truth does not depend on any matter of fact or state of the world, but merely on the relation of concepts within the statement itself. Because an analytic statement is necessarily true, its negation would be self-contradictory, which is the second type of statement Hempel refers to in his first category. For example, the statement “The red ball is not red” contradicts itself and can under no circumstance be true. The same can be said of the statements “All bachelors are married” and “Some bachelors are married.” It is the relationship (one of identity or contradiction) among the concepts included in an analytic or self-contradictory statement that makes it cognitively meaningful.

Synthetic statements are empirically verifiable statements. A synthetic statement is one that brings together (synthesizes) two unrelated concepts. This, of course, is in contradistinction to analytic statements in which the predicate is included in the subject. For example, as noted before, whereas “The red ball is red” is an analytic statement, the statement “The ball is red” would be a synthetic statement. There is no necessary or a priori relationship between the concept “ball” and the concept “red.” The predicate is not included in the subject. Rather, the statement synthesizes two unrelated concepts and asserts a (contingent) relationship between them. This relationship is one that can be verified by empirical observation. In the case of this simple statement, one need only observe the ball in question to verify that it is indeed red. It is also worth noting that the logical positivists intended this standard to be interpreted as “verifiability in principle,” and not merely “practical verifiability” (Ayer, 1952/2000, p. 29). A practically verifiable statement would be one that, given current knowledge and technology, is in fact possible to verify empirically. However, a statement could still be meaningful if it made a claim that was not practically verifiable but was still verifiable in principle. Consider the Planet X hypothesis. This hypothesis is that there is a planet in our solar system that astronomers have so far failed to detect. The reason for this failure is that Planet X shares an orbit with the planet Earth. However, its orbit is such that it is always on the opposite side of the sun. Thus, we can never see this planet from our own. The claim of Planet X’s existence is not a practically verifiable claim. However, if we could build a powerful enough spaceship, we could travel around the sun to verify the existence of this planet, or verify its nonexistence. Thus, the claim of Planet X’s existence is verifiable in principle, and thus is a meaningful statement given logical positivist criteria.

Statements of a metaphysical sort are not analytic and are not subject to this type of “experiential test”—even hypothetically (in principle). Carnap uses the example of statements referring to “god.” As the term god refers to an entity “beyond experience” (Carnap, 1932/1959, p. 66), any claim regarding “god” would not be experientially testable. In other words, such a claim would not be verifiable. Any statement that predicates any quality of “god” could not be said to be true or not true. Such a claim would, hence, be not necessarily false but nonsense. Ayer uses, as an example, a statement from the metaphysical philosopher F. H. Bradley: “the Absolute enters into, but is itself incapable of, evolution and progress” (Ayer, 1952/2000, pp. 29–30). Like metaphysical statements about “god,” this metaphysical statement is not verifiable practically or in principle, because, says Ayer, “one cannot conceive of an observation which would enable one to determine whether the Absolute did, or did not, enter into evolution and progress” (1952/2000, p. 30). Thus, like statements about “God,” this statement, being neither analytic nor empirically verifiable, is nonsense. Hans Reichenbach used a statement from Hegel as an example: “Reason is substance, as well as infinite power, its own infinite material underlying all the natural and spiritual life; as also the infinite form, that which sets the material in motion” (Reichenbach, 1951, p. 3). Such a statement, Reichenbach claimed, could well inspire a reader to cast the offending book into the fire.¹ Logical positivists even refused to refer to statements such as these as “statements” (given that a “statement” should be meaningful, that is, determinable as true or false) and instead referred to them as “pseudo-statements” or “pseudo-propositions” (Ayer, 1952/2000, p. 29; Carnap, 1928/1967, p. 326; 1932/1959, pp. 61, 68, 69–78).

Before we can address the question of what this theory of meaning says about science, there is an issue of internal debate among logical positivists that is worth noting. One can interpret the verifiability principle in two ways, what Ayer (1952/2000) refers to as strong and weak verifiability. Strong verifiability requires the truth of a claim to be, in principle at least, conclusively verifiable, that is, “conclusively established in experience” (Ayer, 1952/2000, p. 30). Under strong verifiability then, only those statements whose truth could be established absolutely and beyond all doubt could be accepted as meaningful. This interpretation sets a rather high bar for meaningfulness. According to Ayer (1952/2000), Moritz Schlick affirmed this high bar. Ayer thought this criterion too restrictive though. Accepting this interpretation would entail rejecting as nonsense many commonly accepted generalities or “general propositions of law” in Ayer’s (1952/2000, p. 30) terms. For example, the claim “All men are mortal” could not be accepted as meaningful under this interpretation of verifiability. The problem is that general claims such as these are not only commonly and intuitively accepted but quite useful also. One cannot prove (in the mathematical or logical sense of the word) that all men are mortal. One can at best prove that all men observed to date have turned out to be mortal. Yet not accepting such a plainly true proposition would seem foolish. Under the strong verifiability principle, general claims would be nonsense much the same as claims about “God” or “the Absolute.” Moritz Schlick negotiated his way around the problem of recognizing the usefulness of general claims while maintaining their status as nonsense under the strong verifiability principle by referring to such claims as “an essentially important type of nonsense” (Ayer, 1952/2000, p. 30).

Ayer favored the weak interpretation of the verifiability principle, describing the strong interpretation as “self-stultifying” (Ayer, 1952/2000, p. 30). He found Schlick’s qualification of general laws as important nonsense as a hedge: a recognition of a paradox created by this restrictive criterion without actually removing the paradox (Ayer, 1952/2000). According to the weak interpretation, a claim is meaningful only if “it is possible for experience to render it probable” (Ayer, 1952/2000, p. 30). Unlike the strong interpretation, a claim does not have to be provable in an absolute or beyond all doubt sense. It only has to be provable to a degree of probability. Thus, general claims and general laws could be accepted as meaningful under this criterion. One may not be able to prove the truth of a claim like “All men are mortal” to an absolute degree of certainty, but one can prove it to a degree of probability, making it meaningful under the weak verifiability principle. The problem with statements like these is that they do refer to something beyond experience just as metaphysical claims do. The claim “All men are mortal” refers not only to all the men who have died (and thus empirically establishing their mortality), but it also refers to all currently living men whose mortality is yet to be established and to all future men, one of whom, it is possible, could be born immortal. Schlick, by maintaining these types of statements as nonsense, retained a sense of consistency but at the cost of commonsense and an asserted paradox: “important nonsense,” whereas Ayer saved common sense and avoided a paradox, and hence he risked charges of inconsistency.

Although the verifiability principle can be taken as a general principle of meaning, it also establishes an important general criterion for scientific methodology and even the meaning of “science” itself. Because the verifiability principle can be interpreted as a general theory of meaning, it cannot act as a sufficient condition for “science.” It, however, does seem a necessary condition. That is, it seems a clear thesis of logical positivism that one cannot have “science” without the verifiability principle as a standard. Otherwise, science would devolve into a morass of pseudostatements and lose meaning, efficaciousness, and authority. “All empirical sciences (natural sciences, psychology, cultural science),” writes Carnap, “acknowledge and carry out in practice the requirement that every statement must have factual content … each statement which is to be considered meaningful in any one of these fields … either goes directly back to experience … or it is at least indirectly connected with experience in such a way that it can be indicated which possible experience would confirm or refute it” (1932/1967, p. 328).

The application of the verifiability principle seems at times normative and at times merely descriptive. The aforementioned quote from Carnap appears to apply the principle descriptively. He seems to be merely stating the way that science does in fact work, what scientists do in fact do. However, it can also be applied normatively, to ridicule the nonsense of studies outside the realm of science, to establish a proper philosophical method or to establish the proper meaning and practice of science so as to exclude mere pseudosciences. Regarding the second of these, the logical positivists applied this criterion to philosophy, not only cutting away the metaphysical philosophy of the likes of Hegel, Bradley, and Heidegger but establishing a stricter, more empirical approach to philosophy sometimes called by them “scientific philosophy” (Carnap, 1932/1959, p. 77; Reichenbach, 1951), which would focus on the logical analysis of language. Regarding the third of these, if a field of study cannot demonstrate a consistent employment and deployment of meaningful terms, it would then not be science—even if it patently made claims to be so.

The strong versus weak interpretations of the verifiability principle become particularly important in the context of science. General claims and laws are central to science. Reichenbach (1951) goes so far as to say that “Generalization … is the origin of science” (p. 5). Newton’s laws of motion are generalizations. The claim that lead is heavier than gold is a generalization. The claim that a particular therapy is helpful in relieving a particular malady is a generalization. We cannot have science without such generalizations. This is likely the prime reason Schlick qualified these nonsense statements (under his interpretation) as “important” nonsense. Not only are they central to daily life and common sense, but they are also central to science. This is likely also why Ayer risked charges of inconsistency by advocating the weak verifiability principle.

This centrality of generalization to science also points to the centrality of inductive logic, as it is inductive logic that generates generalizations. And, of course, an inductive argument can establish its conclusion only to a degree of probability. Knowledge, then, is generated by a mixture of empirical observation and logic. Empirical observation itself only provides observations of particular phenomena. In order to generate genuine scientific claims, we have to apply inductive logic to these observations. Thus, we get empirical observation plus logical inference, which is the essence of science for the logical empiricists: empiricism plus logic.

 POPPER: SCIENCE IS FALSIFIABILITY

Karl Popper (2002) was the first to formally define this issue and coin the term problem of demarcation (p. 11). The logical positivists may not have explicitly identified verifiability as a criterion for science, but in their vigorous attacks on metaphysics they clearly implied such a criterion. And Popper, in his writings on the question, clearly accepted verifiability as a logical positivist criterion for science. But in accepting verifiability as the logical positivist criterion, he strongly rejected it as the correct criterion. In doing so, he also denied the centrality of inductive logic to science in favor of deductive logic and distinguished the question of meaning from the question of demarcation.

Popper’s stated inspiration for embarking on this particular study was the prevalence of a number of theories in the early 20th century. These theories were Einstein’s theory of relativity, Marx’s theory of history, Freud’s psychoanalysis, and Alfred Adler’s individual psychology (Popper, 1963/2000, p. 9). Whereas all four of these theories at the time were receiving a lot of attention from some very smart people, to Popper, Einstein’s theory seemed different from the other three. To him, those three appeared more mythical than scientific, more like astrology than astronomy. He then set out to determine what quality or qualities precisely defined this distinction. The first step in this process was to evaluate the logical positivist answer. He found verifiability inadequate as a criterion for demarcation. It is theoretically illiberal, according to Popper, because the “positivists, in their anxiety to annihilate metaphysics, annihilate natural science along with it” (Popper, 2002, p. 13). He makes explicit reference to Schlick and his use of the strong verifiability principle. Recall the problems that the principle had with general laws. Under the strong verifiability principle it becomes impossible to verify as seemingly simple a scientific claim as “arsenic is poisonous.” Such a claim depends on inductive logic, as does the verifiability principle itself. Inductively, it is impossible to conclusively prove that arsenic is poisonous. To do so, one would have to test every sample of arsenic. One might even have to test every sample on every living being, or at least on every human being if we could delimit the arsenic claim in such a way. Not only did Popper (2002) reject the verifiability principle as a criterion of demarcation, he further rejected induction as a form of logic. In this way he might be said to more consistently follow the ideas of Hume than the logical positivists.

Regarding the theories of Marx, Freud, and Adler, the problem is that they do seem verifiable. These theorists and their followers can point to many confirming instances. Here we get the counterintuitive quality of Popper’s position. Confirming or verifying a theory does not really show it to be true. This is due in part to the limits of induction, but there is another related problem as well. Of psychoanalysis and individual psychology specifically, Popper wrote:

I could not think of any human behavior which could not be interpreted in terms of either theory. It was precisely this fact—that they always fitted, that they were always confirmed—which in the eyes of their admirers constituted the strongest argument in favor of these theories. It began to dawn on me that this apparent strength was in fact their weakness. (1963/2000, p. 10)

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