The principle central to logical positivism, according to which the meaning of a statement is its method of verification. Sentences apparently expressing propositions that admit of no verification (such as those of metaphysics and theology) are in consequence meaningless, or at least fail to put forward theses with cognitive meaning, capable of truth or falsity. The principle requires confidence that we know what a verification consists in, and tended to coexist with a fairly simple conception of each thought as answerable to individual experiences. To avoid undue simplification the principle moved from requiring a strong or conclusive verification as the condition of meaning, to admitting indirect and inconclusive methods of verification. However, more complex and holistic conceptions of language and its relation to the world suggest a more flexible set of possible relations, with sentences that are individually not verifiable nevertheless having a use in an overall network of beliefs or theory that itself answers to experience.