logical positivism also called positivism, a philosophical movement inspired by empiricism and verificationism; it began in the 1920s and flourished for about twenty or thirty years. While there are still philosophers who would identify themselves with some of the logical positivists’ theses, many of the central docrines of the theory have come under considerable attack in the last half of this century. In some ways logical positivism can be seen as a natural outgrowth of radical or British empiricism and logical atomism. The driving force of positivism may well have been adherence to the verifiability criterion for the meaningfulness of cognitive statements. Acceptance of this principle led positivists to reject as problematic many assertions of religion, morality, and the kind of philosophy they described as metaphysics. The verifiability criterion of meaning. The radical empiricists took genuine ideas to be composed of simple ideas traceable to elements in experience. If this is true and if thoughts about the empirical world are ‘made up’ out of ideas, it would seem to follow that all genuine thoughts about the world must have as constituents thoughts that denote items of experience. While not all positivists tied meaning so clearly to the sort of experiences the empiricists had in mind, they were convinced that a genuine contingent assertion about the world must be verifiable through experience or observation. Questions immediately arose concerning the relevant sense of ‘verify’. Extreme versions of the theory interpret verification in terms of experiences or observations that entail the truth of the proposition in question. Thus for my assertion that there is a table before me to be meaningful, it must be in principle possible for me to accumulate evidence or justification that would guarantee the existence of the table, which would make it impossible for the table not to exist. Even this statement of the view is ambiguous, however, for the impossibility of error could be interpreted as logical or conceptual, or something much weaker, say, causal. Either way, extreme verificationism seems vulnerable to objections. Universal statements, such as ‘All metal expands when heated’, are meaningful, but it is doubtful that any observations could ever conclusively verify them. One might modify the criterion to include as meaningful only statements that can be either conclusively confirmed or conclusively disconfirmed. It is doubtful, however, that even ordinary statements about the physical world satisfy the extreme positivist insistence that they admit of conclusive verification or falsification. If the evidence we have for believing what we do about the physical world consists of knowledge of fleeting and subjective sensation, the possibility of hallucination or deception by a malevolent, powerful being seems to preclude the possibility of any finite sequence of sensations conclusively establishing the existence or absence of a physical object.
Faced with these difficulties, at least some positivists retreated to a more modest form of verificationism which insisted only that if a proposition is to be meaningful it must be possible to find evidence or justification that bears on the likelihood of the proposition’s being true. It is, of course, much more difficult to find counterexamples to this weaker form of verificationism, but by the same token it is more difficult to see how the principle will do the work the positivists hoped it would do of weeding out allegedly problematic assertions.
Necessary truth. Another central tenet of logical positivism is that all meaningful statements fall into two categories: necessary truths that are analytic and knowable a priori, and contingent truths that are synthetic and knowable only a posteriori. If a meaningful statement is not a contingent, empirical statement verifiable through experience, then it is either a formal tautology or is analytic, i.e., reducible to a formal tautology through substitution of synonymous expressions. According to the positivist, tautologies and analytic truths that do not describe the world are made true (if true) or false (if false) by some fact about the rules of language. ‘P or not-P’ is made true by rules we have for the use of the connectives ‘or’ and ‘not’ and for the assignments of the predicates ‘true’ and ‘false’.
Again there are notorious problems for logical positivism. It is difficult to reduce the following apparently necessary truths to formal tautologies through the substitution of synonymous expressions: (1) Everything that is blue (all over) is not red (all over). (2) All equilateral triangles are equiangular triangles. (3) No proposition is both true and false. Ironically, the positivists had a great deal of trouble categorizing the very theses that defined their view, such as the claims about meaningfulness and verifiability and the claims about the analytic–synthetic distinction. Reductionism. Most of the logical positivists were committed to a foundationalist epistemology according to which all justified belief rests ultimately on beliefs that are non-inferentially justified. These non-inferentially justified beliefs were sometimes described as basic, and the truths known in such manner were often referred to as self-evident, or as protocol statements. Partly because the positivists disagreed as to how to understand the notion of a basic belief or a protocol statement, and even disagreed as to what would be good examples, positivism was by no means a monolithic movement. Still, the verifiability criterion of meaning, together with certain beliefs about where the foundations of justification lie and beliefs about what constitutes legitimate reasoning, drove many positivists to embrace extreme forms of reductionism. Briefly, most of them implicitly recognized only deduction and (reluctantly) induction as legitimate modes of reasoning. Given such a view, difficult epistemological gaps arise between available evidence and the commonsense conclusions we want to reach about the world around us. The problem was particularly acute for empiricists who recognized as genuine empirical foundations only propositions describing perceptions or subjective sensations. Such philosophers faced an enormous difficulty explaining how what we know about sensations could confirm for us assertions about an objective physical world. Clearly we cannot deduce any truths about the physical world from what we know about sensations (remember the possibility of hallucination). Nor does it seem that we could inductively establish sensation as evidence for the existence of the physical world when all we have to rely on ultimately is our awareness of sensations. Faced with the possibility that all of our commonplace assertions about the physical world might fail the verifiability test for meaningfulness, many of the positivists took the bold step of arguing that statements about the physical world could really be viewed as reducible to (equivalent in meaning to) very complicated statements about sensations. Phenomenalists, as these philosophers were called, thought that asserting that a given table exists is equivalent in meaning to a complex assertion about what sensations or sequences of sensations a subject would have were he to have certain other sensations. The gap between sensation and the physical world is just one of the epistemic gaps threatening the meaningfulness of commonplace assertions about the world. If all we know about the mental states of others is inferred from their physical behavior, we must still explain how such inference is justified. Thus logical positivists who took protocol statements to include ordinary assertions about the physical world were comfortable reducing talk about the mental states of others to talk about their behavior; this is logical behaviorism. Even some of those positivists who thought empirical propositions had to be reduced ultimately to talk about sensations were prepared to translate talk about the mental states of others into talk about their behavior, which, ironically, would in turn get translated right back into talk about sensation.
Many of the positivists were primarily concerned with the hypotheses of theoretical physics, which seemed to go far beyond anything that could be observed. In the context of philosophy of science, some positivists seemed to take as unproblematic ordinary statements about the macrophysical world but were still determined either to reduce theoretical statements in science to complex statements about the observable world, or to view theoretical entities as a kind of convenient fiction, description of which lacks any literal truth-value. The limits of a positivist’s willingness to embrace reductionism are tested, however, when he comes to grips with knowledge of the past. It seems that propositions describing memory experiences (if such ‘experiences’ really exist) do not entail any truths about the past, nor does it seem possible to establish memory inductively as a reliable indicator of the past. (How could one establish the past correlations without relying on memory?) The truly hard-core reductionists actually toyed with the possibility of reducing talk about the past to talk about the present and future, but it is perhaps an understatement to suggest that at this point the plausibility of the reductionist program was severely strained.
See also ANALYTIC – SYNTHETIC DISTINC – TION , BEHAVIORISM , EMPIRICISM , FOUNDA – TIONALISM , PHILOSOPHY OF SCIENCE , VERIFI – CATIONISM , VIENNA CIRCL. R.A.F.