The analytic–synthetic distinction is a semantic distinction used primarily in philosophy to distinguish between propositions (in particular, statements that are affirmative subject–predicate judgments) that are of two types: analytic propositions and synthetic propositions. Analytic propositions are true or not true solely by virtue of their meaning, whereas synthetic propositions' truth, if any, derives from how their meaning relates to the world. While the distinction was first proposed by Immanuel Kant, it was revised considerably over time, and different philosophers have used the terms in very different ways. Furthermore, some philosophers (starting with Willard Van Orman Quine) have questioned whether there is even a clear distinction to be made between propositions which are analytically true and propositions which are synthetically true. Debates regarding the nature and usefulness of the distinction continue to this day in contemporary philosophy of language. The philosopher Immanuel Kant uses the terms "analytic" and "synthetic" to divide propositions into two types. Kant introduces the analytic–synthetic distinction in the Introduction to his Critique of Pure Reason (1781/1998, A6–7/B10–11). There, he restricts his attention to statements that are affirmative subject–predicate judgments and defines "analytic proposition" and "synthetic proposition" as follows: analytic proposition: a proposition whose predicate concept is contained in its subject concept synthetic proposition: a proposition whose predicate concept is not contained in its subject concept but related Examples of analytic propositions, on Kant's definition, include: "All bachelors are unmarried." "All triangles have three sides." Kant's own example is: "All bodies are extended": that is, they occupy space. (A7/B11) Each of these statements is an affirmative subject–predicate judgment, and, in each, the predicate concept is contained within the subject concept. The concept "bachelor" contains the concept "unmarried"; the concept "unmarried" is part of the definition of the concept "bachelor".