Recent Western Philosophy I: Frege, Logical Positivism, Moore and Russell

Logical positivism was the programme of the Vienna Circle of the 1920s and 1930s (Moritz Schlick, Rudolf Carnap, Otto Neurath, Herbert Feigl, Friedrich Waismann), brought to the English-speaking world by A. J. Ayer's Language, Truth and Logic (1936). It combined Hume's division of knowledge with the new logic of Frege, Russell and the early Wittgenstein.

• The verification theory of meaning: "the meaning of a proposition is the method of its verification" (Schlick). A sentence is literally meaningful only if it is either analytic (true by definition, like logic and mathematics) or empirically verifiable. Ayer distinguished strong verification (conclusive establishment, which no general law allows) from weak verification (experience can make it probable), and verifiability in principle from verifiability in practice ("there are mountains on the far side of the moon" was meaningful before anyone could go there).
• Elimination of metaphysics: metaphysical sentences ("the Absolute enters into, but is itself incapable of, evolution and progress", Ayer's example from Bradley) are neither analytic nor verifiable, so they are not false but meaningless pseudo-propositions. Carnap's essay "The Elimination of Metaphysics through Logical Analysis of Language" (1932) made Heidegger's "the Nothing itself nothings" its example. The same fate befalls theology and, for Ayer, the non-descriptive part of ethics (emotivism).
• Concept of philosophy: philosophy is not a body of doctrines competing with science but an activity of clarifying meaning. Schlick's "The Turning Point in Philosophy" (1930) says science pursues truth and philosophy meaning; for Ayer philosophy is "a department of logic", concerned with definitions and the logical relations of propositions, not with facts about a supersensible reality.

• "The Refutation of Idealism" (1903): idealism rests on esse est percipi. Moore argues that every sensation contains two elements, the act of consciousness, which is common to the sensation of blue and the sensation of green, and the object, which differs (blue or green). Idealists confuse the two; once they are distinguished, there is no contradiction in the object existing when it is not experienced, and the claim that to be is to be perceived loses its support.
• "A Defence of Common Sense" (1925): Moore lists truisms he knows with certainty: that there exists a living human body which is his, that it was born some time ago, that the earth existed for many years before his body was born, that other human beings have had similar experiences. Philosophers who deny such things contradict themselves, since they speak of "we" and of other philosophers. The problem is not whether such propositions are true but what their correct analysis is.
• "Proof of an External World" (1939): holding up his hands, Moore says "here is one hand" and "here is another", so at least two external objects exist, so there is an external world. The proof meets his conditions for a rigorous proof: the premises differ from the conclusion, he knows them to be true, and the conclusion follows. Critics said it begs the question against the sceptic; Moore's point is that his knowledge of the premises is more certain than any sceptical argument against them.
• Sense-data: in lectures of 1910–11 (published as Some Main Problems of Philosophy, 1953) and later papers, Moore held that when I see my hand I directly apprehend a sense-datum, and that the analysis of "this is a hand" involves asking how that sense-datum is related to the surface of the hand. He considered three answers, that it is identical with part of the surface, that it is a distinct entity standing in some ultimate relation to the surface, and a phenomenalist reduction, and never settled among them.

In "On Denoting" (1905), Bertrand Russell (1872–1970) analysed definite descriptions. "The present King of France is bald" seems to be about something that does not exist. Russell's theory says the description is not a name but an "incomplete symbol" that disappears on analysis: the sentence means that there is at least one present King of France, there is at most one, and whoever is present King of France is bald. In symbols, ∃x(Kx ∧ ∀y(Ky → y = x) ∧ Bx). Since the first clause is false, the whole sentence is false, not meaningless.

• What the theory solves: negative existentials ("the golden mountain does not exist" is true without there being a golden mountain to deny); informative identity ("Scott is the author of Waverley", which King George IV wished to know, is not the trivial "Scott is Scott"); and excluded middle, by distinguishing a primary occurrence of the description ("there is a present King of France who is not bald", false) from a secondary one ("it is not the case that the present King of France is bald", true).
• Logical atomism ("The Philosophy of Logical Atomism", lectures of 1918): the world consists of facts, not things; the simplest are atomic facts, a particular having a quality or particulars standing in a relation, pictured by atomic propositions. Molecular propositions ("p or q") are truth-functions of atomic ones. The ultimate constituents are known by acquaintance, and the only logically proper names are words like "this" and "that" for present sense-data; ordinary names are disguised descriptions (Russell's 1910–11 distinction between knowledge by acquaintance and knowledge by description).
• Refutation of idealism: around 1898 Russell and Moore broke with the British idealism of Bradley and McTaggart. Russell rejected the doctrine of internal relations, that every relation is grounded in the natures of its terms, which leads to monism; relations are external, and so plurality and asymmetrical relations (needed for mathematics) are real. In The Problems of Philosophy (1912, chapter 4) he argued that Berkeley's case trades on an ambiguity in "idea" between the act of apprehending and the thing apprehended.