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Mauro Murzi's pages on Philosophy of Science - Logical Positivism
Verifiability Principle
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2. The Main Philosophical Tenets of Logical Positivism.


a. Verifiability Principle.

According to logical positivism, there are only two sources of knowledge: logical reasoning and empirical experience. The former is analytic a priori, while the latter is synthetic a posteriori; hence synthetic a priori knowledge does not exist.

It is precisely in the rejection of the possibility of synthetic knowledge a priori that the basic thesis of modern empiricism lies. (Wissenschaftliche Weltauffassung. Der Wiener Kreis, 1929; English translation The Scientific Conception of the World. The Vienna Circle, in Sarkar, Sahotra (ed.), The Emergence of Logical Empiricism: from 1900 to the Vienna Circle, New York: Garland Publishing, 1996, p. 330).

Logical knowledge includes mathematics, which is claimed to be reducible to formal logic. Empirical knowledge includes physics, biology, psychology, etc. Experience is the only judge of scientific theories; however, logical positivists were aware that scientific knowledge does not exclusively rise from the experience: scientific theories are genuine hypotheses that go beyond the experience.

It is not possible to establish a logically durable building on verifications [a verification is an observational statement about immediate perception], for they are already vanished when the building begins. If they were, with respect to time, at the beginning of the knowledge, then they would be logically useless. On the contrary, there is a great difference when they are at the end of the process: with their help the test is performed.... From a logical point of view, nothing depends on them: they are not premises but a firm end point. (M. Schlick, "Über das Fundament der Erkenntnis", in Erkenntnis, 4, 1934).

A statement is meaningful if and only if it can be proved true or false, at least in principle, by means of the experience.  This assertion is called the Verifiability Principle. It follows that the meaning of a statement is its method of verification; that is, we know the meaning of a statement if we know the conditions under which the statement is true or false.

When are we certain, in general, that the meaning of a question is clear to us? Obviously then, and only then, when we are in a position to state quite accurately the circumstances under which it can be answered in the affirmative - or those under which it would have to receive a negative answer. By these statements, and these alone, is the meaning of the question defined.... To state the circumstances under which a proposition is true is the same as stating its meaning, and nothing else.... a statement only has a specifiable meaning if it makes some testable difference whether it is true or false. (M. Schlick, "Positivismus und Realismus" in Erkenntnis, 3, 1932; English translations "Positivism and Realism" in Sarkar, Sahotra (ed.), Logical Empiricism at its Peak: Schlick, Carnap, and Neurath, New York: Garland Pub., 1996).

Metaphysical statements are thus forbidden: they are meaningless according to the Verifiability Principle. Also, traditional philosophy is indeed meaningless, and the only role of philosophy is the clarification of the meaning of statements.

Philosophy, in fact, is that activity whereby the meaning of statements is established or discovered.  (M. Schlick, "Die Wende der Philosophie" in Erkenntnis, 1, 1930; English translation in "The Turning-Point in Philosophy" Sarkar, Sahotra (ed.), Logical Empiricism at its Peak: Schlick, Carnap, and Neurath, New York: Garland Pub., 1996, p. 5).

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