[2] The verifiability criterion thus rejected statements of metaphysics, theology, ethics and aesthetics as cognitively meaningless in terms of truth value or factual content.
By the 1950s, problems identified within logical positivism's central tenets became seen as intractable, drawing escalating criticism among leading philosophers, notably from Willard van Orman Quine and Karl Popper, and even from within the movement, from Carl Hempel.
[11] Early in the movement, Carnap, Hahn, Neurath and others recognised that the verifiability criterion was too stringent in that it rejected universal statements, which are vital to scientific hypothesis.
[12] A radical left wing emerged from the Vienna Circle, led by Neurath and Carnap, who proposed revisions to weaken the criterion, a program they referred to as the "liberalisation of empiricism".
A conservative right wing, led by Schlick and Waismann, instead sought to classify universal statements as analytic truths, thereby to reconcile them with the existing criterion.
[12] Both Schlick and Carnap had been influenced by and sought to define logical positivism versus the neo-Kantianism of Ernst Cassirer, the contemporary leading figure of the Marburg school, and against Edmund Husserl's phenomenology.
[14] By the late 1930s, many in the movement had replaced phenomenalism with Neurath's physicalism, whereby material objects are not reducible to sensory stimuli but exist as publicly observable entities in the real world.
At the same time, the movement drew intensifying scrutiny over its central problems[16][17][18] and its doctrines were increasingly criticised, most trenchantly by Willard Van Orman Quine, Norwood Hanson, Karl Popper, Thomas Kuhn and Carl Hempel.
Russell then continued it with Alfred North Whitehead in their Principia Mathematica, inspiring some of the more mathematical logical positivists, such as Hans Hahn and Rudolf Carnap.
[23][24] Immanuel Kant identified a further category of knowledge: Synthetic a priori statements, which affirm a state of facts concerning the world, but are knowable without observation.
[25] His thesis would serve to rescue Newton's law of universal gravitation from Hume's problem of induction by determining uniformity of nature to be in the category of a priori knowledge.
[33] Cognitive meaningfulness was defined variably: possessing truth value; or corresponding to a possible state of affairs; or intelligible or understandable as are scientific statements.
Specifically, universal statements were noted to be empirically unverifiable, rendering vital domains of science and reason, such as scientific hypothesis, cognitively meaningless under verificationism.
[37] The logical positivist movement shed much of its revolutionary zeal following the defeat of Nazism and the decline of rival philosophies that sought radical reform, notably Marburg neo-Kantianism, Husserlian phenomenology and Heidegger's existential hermeneutics.
[45] According to the DN model, a scientific explanation is valid only if it takes the form of a deductive inference from a set of explanatory premises (explanans) to the observation or theory to be explained (explanandum).
This would become sustained from various directions by the 1950s,[12] so that, even among fractious philosophers who disagreed on the general objectives of epistemology, most would concur that the logical positivist program had become untenable.
[54] Notable critics included Karl Popper, W. V. O. Quine, Norwood Hanson, Thomas Kuhn, Hilary Putnam,[6] as well as J. L. Austin, Peter Strawson, Nelson Goodman and Richard Rorty.
Any attempt to do so, he argued, would commit the fallacy of affirming the consequent, given that verification cannot—in itself—exclude alternative valid explanations for a specific phenomenon or instance of observation.
[60] In rejecting neo-positivist views of cognitive meaningfulness, Popper considered metaphysics to be rich in meaning and important in the origination of scientific theories and value systems to be integral to science's quest for truth.
At the same time, he disparaged pseudoscience, referring to the confirmation biases that embolden support for unfalsifiable conjectures (notably those in psychology and psychoanalysis) and ad hoc arguments used to entrench predictive theories that have been proven conclusively false.
[59] In his influential 1951 paper Two Dogmas of Empiricism, American philosopher and logicist Willard Van Orman Quine challenged the analytic-synthetic distinction.
Specifically, Quine examined the concept of analyticity, determining that all attempts to explain the idea reduce ultimately to circular reasoning.
Hanson and Thomas Kuhn held that even direct observations are never truly neutral in that they are laden with theory, i.e. influenced by a system of theoretical presuppositions that function as an interpretative framework for the senses.
[66] Kuhn proposed in its place a coherentist model of science, whereby scientific progress revolves around cores of established, coherent ideas which periodically undergo abrupt revolutionary changes.
[67] Though foundationalism was often considered a constituent doctrine of logical positivism (and Kuhn's thesis an epistemological criticism of the movement) such views were simplistic:[68] In the 1930s, Neurath had argued for the adoption of coherentism, famously comparing the progress of science to reconstruction of a boat at sea.
The conservative wing of the Vienna Circle under Moritz Schlick subscribed to a form of foundationalism, but its principles were defined unconventionally or ambiguously.
[70] In some sense, Kuhn's book unified science, but through historical and social assessment rather than by networking the scientific specialties using epistemological or linguistic models.
[71] His ideas were adopted quickly by scholars in non-scientific disciplines,[71] such as the social sciences in which neo-positivists were dominant,[44] ushering academia into postpositivism or postempiricism.
He also spurned instrumentalism, according to which a scientific theory is judged, not by whether it corresponds to reality, but by the extent to which it allows empirical predictions or resolves conceptual problems.
[76] Logical positivism's fall heralded postpositivism, distinguished by Popper's critical rationalism—which characterised human knowledge as continuously evolving via conjectures and refutations—and Kuhn's historical and social perspectives on the saltatory course of scientific progress.