Logical positivism (later referred to as logical empiricism) holds that philosophy should aspire to the same sort of rigor as science. Philosophy should provide strict criteria for judging sentences true, false and meaningless. The most characteristic claim of logical positivism asserts that statements are meaningful only insofar as they are verifiable, and that statements can be verified only in two (exclusive) ways: empirical statements, including scientific theories, which are verified by experiment and evidence; and analytic truth, statements which are true or false by definition, and so are also meaningful. Everything else, including ethics and aesthetics, is not literally meaningful, and so belonged to "metaphysics." One conclusion is that serious philosophy should no longer concern itself with metaphysics.
Logical positivism was one of the early manifestations of analytic philosophy. It was the philosophical position of the Vienna Circle in its early years, and gained recognition in the English speaking world through the work of A. J. Ayer. The term subsequently came to be almost interchangeable with "analytic philosophy" in the first half of the twentieth century. Logical Positivism was immensely influential in philosophy of science, logic, and philosophy of language. Even though few of its tenets are still agreed with, its role in forming contemporary philosophy should not be underestimated; many subsequent commentators on "logical positivism" tend to attribute to it more of a singular purpose and creed than it in fact adhered to, overlooking the complex disagreements among the logical positivists themselves. Under this view, statements about God, good and evil, and beauty are neither true nor false, and thus should not be taken seriously. Positivism was the dominant theory of the philosophy of science between World War I and the Cold War.
Logical positivism took up the projects of Bertrand Russell and the early Ludwig Wittgenstein (who, along with Albert Einstein, were held up by the circle as the paragons of modern science and philosophy).
Critics of Logical Positivism say that its fundamental tenets could not themselves be formulated in a way that was clearly consistent. The verifiability criterion of meaning did not seem verifiable; but neither was it simply a logical tautology, since it had implications for the practice of science and the empirical truth of other statements. This presented severe problems for the logical consistency of the theory. Another problem was that, while positive existential claims (There is at least one human being) and negative universals (Not all ravens are black) allow for clear methods of verification (find a human or a non-black raven), negative existential claims and positive universal claims do not.
Universal claims could apparently never be verified: How can you tell that all ravens are black, unless you've hunted down every raven ever, including those in the past and future? This led to a great deal of work on induction, probability, and "confirmation," (which combined verification and falsification; see below).
Karl Popper, a well known critic of Logical Positivism, published the book Logik der Forschung (Eng.:The Logic of Scientific Discovery) in 1934. In it he presented an influential alternative to the verifiability criterion of meaning, explaining scientific statements in terms of falsifiability. First, though, Popper's concern was not with distinguishing meaningful from metaphysical (meaningless) statements, but distinguishing science from pseudo-science. He did not hold that metaphysical statements must be meaningless; in some cases, such as Marxism, he held that they were meaningful and had been falsified. In others, such as psychoanalysis, he held that they offered no method for falsification, and so were not science. He was, in general, more concerned with scientific practice than with the logical issues that troubled the positivists. Second, although Popper's philosophy of science enjoyed great popularity for some years, if his criterion is construed as an answer to the question the positivists were asking it turns out to fail in exactly parallel ways. Negative existential claims (There are no unicorns) and positive universals (All ravens are black) can be falsified, but positive existential and negative universal claims cannot.
Logical positivists' response to the first criticism is that Logical Positivism, like all other philosophies of science, is a philosophy of science, not an axiomatic system that can prove its own consistency. (see Gödel's incompleteness theorem) Secondly, a theory of language and mathematical logic were created to answer what it really means to say things like "all ravens are black".
Subsequent philosophy of science tends to make use of the better aspects of both of these approaches. Work by W. V. Quine and Thomas Kuhn has convinced many that it is not possible to provide a strict criterion for good or bad scientific method outside of the science we already have. But even this sentiment was not unknown to the logical positivists: Otto Neurath famously compared science to a boat which we must rebuild on the open sea.
- Logik der Forschung in 1934