Logical positivism
Logical positivism, also known as logical empiricism or neo-positivism, was a philosophical movement, in the empiricist tradition, that sought to formulate a scientific philosophy in which philosophical discourse would be, in the perception of its proponents, as authoritative and meaningful as empirical science.
Logical positivism's central thesis was the verification principle, also known as the "verifiability criterion of meaning", according to which a statement is cognitively meaningful only if it can be verified through empirical observation or if it is a tautology. The verifiability criterion thus rejected statements of metaphysics, theology, ethics and aesthetics as cognitively meaningless in terms of truth value or factual content. Despite its ambition to overhaul philosophy by mimicking the structure and process of empirical science, logical positivism became erroneously stereotyped as an agenda to regulate the scientific process and to place strict standards on it.
The movement emerged in the late 1920s among philosophers, scientists and mathematicians congregated within the Vienna Circle and Berlin Circle and flourished in several European centres through the 1930s. By the end of World War II, many of its members had settled in the English-speaking world and the project shifted to less radical goals within the philosophy of science.
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. These problems would remain unresolved, precipitating the movement's eventual decline and abandonment by the 1960s. In 1967, philosopher John Passmore pronounced logical positivism "dead, or as dead as a philosophical movement ever becomes".
Origins
Logical positivism emerged in Germany and Austria amid a cultural background characterised by the dominance of Hegelian metaphysics and the work of Hegelian successors such as F. H. Bradley, whose metaphysics portrayed the world without reference to empirical observation. The late 19th century also saw the emergence of neo-Kantianism as a philosophical movement, in the rationalist tradition.The logical positivist program established its theoretical foundations in the empiricism of David Hume, Auguste Comte and Ernst Mach, along with the positivism of Comte and Mach, defining its exemplar of science in Einstein's general theory of relativity. In accordance with Mach's phenomenalism, whereby material objects exist only as sensory stimuli rather than as observable entities in the real world, logical positivists took all scientific knowledge to be only sensory experience. Further influence came from Percy Bridgman's operationalism—whereby a concept is not knowable unless it can be measured experimentally—as well as Immanuel Kant's perspectives on aprioricity.
Ludwig Wittgenstein's Tractatus Logico-Philosophicus established the theoretical foundations for the verifiability principle. His work introduced the view of philosophy as "critique of language", discussing theoretical distinctions between intelligible and nonsensical discourse. Tractatus adhered to a correspondence theory of truth, as opposed to a coherence theory of truth. Logical positivists were also influenced by Wittgenstein's interpretation of probability though, according to Neurath, some objected to the metaphysics in Tractatus.
History
Vienna and Berlin Circles
The Vienna Circle was led principally by Moritz Schlick, congregating around the University of Vienna and at the Café Central. A manifesto written by Otto Neurath, Hans Hahn and Rudolf Carnap in 1929 summarised the Vienna Circle's positions. Schlick had originally held a neo-Kantian position, but later converted, via Carnap's 1928 book Der logische Aufbau der Welt. The Viennese maintained closely cooperative ties with the Berlin Circle, among whom Hans Reichenbach was pre-eminent. Carl Hempel, who studied under Reichenbach in Germany, was also to prove influential in the movement's later history. A friendly but tenacious critic of the movement was Karl Popper, whom Neurath nicknamed the "Official Opposition".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. 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. Within the liberal wing Carnap emphasised fallibilism, as well as pragmatics, which he considered integral to empiricism. Neurath prescribed a move from Mach's phenomenalism to physicalism, though this would be opposed by Schlick. As Neurath and Carnap sought to pose science toward social reform, the split in the Vienna Circle also reflected political differences.
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. Logical positivists especially opposed Martin Heidegger's obscure metaphysics, the epitome of what they had rejected through their epistemological doctrines. In the early 1930s, Carnap debated Heidegger over "metaphysical pseudosentences".
Anglosphere
As the movement's first emissary to the New World, Moritz Schlick visited Stanford University in 1929, yet otherwise remained in Vienna and was murdered in 1936 at the University by a former student, Johann Nelböck, who was reportedly deranged. That year, A. J. Ayer, a British attendee at various Vienna Circle meetings since 1933, published Language, Truth and Logic, which imported logical positivism to the English-speaking world. In 1933, the Nazi Party's rise to power in Germany had triggered flight of intellectuals, which accelerated upon Germany's annexation of Austria in 1938. The logical positivists, many of whom were Jewish, were targeted and continued flight throughout the pre-war period. Their philosophy thus became dominant in the English-speaking world.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. Neurath settled in England, where he died in 1945. Carnap, Reichenbach and Hempel settled permanently in America.
Post-war period
Following the Second World War, logical positivism—now referred to by some as logical empiricism—turned to less radical objectives in the philosophy of science. Led by Carl Hempel, who expounded the covering law model of scientific explanation, the movement became a major underpinning of analytic philosophy in the English-speaking world and its influence extended beyond philosophy into the social sciences. At the same time, the movement drew intensifying scrutiny over its central problems and its doctrines were increasingly criticised, most trenchantly by Willard Van Orman Quine, Norwood Hanson, Karl Popper, Thomas Kuhn and Carl Hempel.Principles
Verification and Confirmation
Verifiability Criterion of Meaning
According to the verifiability criterion of meaning, a statement is cognitively meaningful only if it is either verifiable by empirical observation or is an analytic truth. Cognitive meaningfulness was defined variably: possessing truth value; or corresponding to a possible state of affairs; or intelligible or understandable as are scientific statements. Other types of meaning—for instance, emotive, expressive or figurative—were dismissed from further review.Metaphysics, theology, as well as much of ethics and aesthetics failed this criterion, and so were found cognitively meaningless and only emotively meaningful. Ethics and aesthetics were considered subjective preferences, while theology and metaphysics contained "pseudostatements" that were neither true nor false. Thus, logical positivism indirectly asserted Hume's law, the principle that factual statements cannot justify evaluative statements, and that the two are separated by an unbridgeable gap. A. J. Ayer's Language, Truth and Logic presented an extreme version of this principle—the boo/hooray doctrine—whereby all evaluative judgments are merely emotional reactions.
Revisions to the criterion
Logical positivists in the Vienna Circle recognised quickly that the verifiability criterion was too restrictive. Specifically, universal statements were noted to be empirically unverifiable, rendering vital domains of science and reason, such as scientific hypothesis, cognitively meaningless under verificationism. This would pose significant problems for the logical positivist program, absent revisions to its criterion of meaning.In his 1936 and 1937 papers, Testability and Meaning, Carnap proposed confirmation in place of verification, determining that, though universal laws cannot be verified, they can be confirmed. Carnap employed abundant logical and mathematical tools to research an inductive logic that would account for probability according to degrees of confirmation. However, he was never able to formulate a model. In Carnap's inductive logic, a universal law's degree of confirmation was always zero. The formulation of what eventually came to be called the "criterion of cognitive significance", stemming from this research, took three decades. Carl Hempel, who became a prominent critic of the logical positivist movement, elucidated the paradox of confirmation.
In his 1936 book, Language, Truth and Logic, A. J. Ayer distinguished strong and weak verification. He stipulated that, "A proposition is said to be verifiable, in the strong sense of the term, if, and only if, its truth could be conclusively established by experience", but is verifiable in the weak sense "if it is possible for experience to render it probable". He would add that, "no proposition, other than a tautology, can possibly be anything more than a probable hypothesis". Thus, he would conclude that all are open to weak verification.