For the philosophers in Austria and Germany who saw natural science as the champion candidate for the title of knowledge, the dangers of the metaphysics of Absolute idealism were made manifestly obvious by the debacle of WWI and its aftermath. Thus they sought to drive a strong separation between metaphysical pronouncements about "Reality" and "Truth" from the sort of statements made by the natural sciences. They saw this difference as lying in the fact that scientific statements were "verifiable" (at least in principle) by empirical evidence. This led the original positivists to rally around the "verifiability criterion of meaning."

The verifiability criterion of meaning

All meaningful statements are either
a) logically analytic (i.e. tautologies true in all possible worlds) or
b) if synthetic, they are verifiable by possible observations (in principle).

Those emipirical facts about the world which would verify a statement are referred to as the Truth Conditions of that statement.

Since metaphysics falls into neither category (according to the positivists), it is cognitively meaningless (though it may give vent to some sort of emotions).

Ethical statements are either true synthetic statements knowable only empirically because the are purely descriptive claims about what people in fact do consider to be "good" or "right" (as established empirically) or they are emotive expressions of approval or disapproval rather than congitive claims.

Taking cues from Russell and Wittgenstein, the positivists held that while most of science consists of general statements of theories, and hence are not directly verifiable by any experiences (which are always particular), these theories or hypotheses have been indirectly verified by observation statements which are directly verified by (or in) experience.

Some positivists thought these observation statements were about sense data, in the way advocated by Russell and others, but the mature development of positivism came to regard observation statements as effectively about the physical objects of the everyday world of experience, a view known as "physicalism." The scientist in effect directly verifies the observational evidence for hypotheses by looking at the laboratory (or other) equipment which provide the data by which hypotheses are verified. Thus positivism began also as a hard-nosed foundationalism based on a correspondence theory of truth. Scientific knowledge is real knowledge because it is verifiable, whereas metaphysics is not verifiable and thus "nonsense." Eventually however this foundationalism began to crumble in a way which pushed positivism towards pragmatism, conventionalism (constructivism), and fallibilism.