synthetic

a statement or item of knowledge which is known to be true because of its connection with some intuition. (Cf. analytic.)

[A9/B13] Opposed to analytic; usually connected with empirical. Unlike analytic judgments, in synthetic judgments the predicate is not "contained within" the subject, but bears some other relation to it. Namely, in empirical synthetic judgments the subject and predicate "belong to one another...continently...as parts of a whole, namely [the whole of] an experience". However, Kant also recognizes synthetic a priori judgments, in which the subject are predicate are related with necessity (not contingently, as in empirical judgments), though the source of this necessity is not (as is the case in analytic judgments) the relation of identity. The source of this ampliative but necessary connection between the predicate and the subject grounds the possibility of synthetic a priori judgments. But what is this source? Normally such ampliative connections are supplied by experience; as Kant puts it, "But in a priori synthetic judgments this help is entirely lacking....Upon what, then, am I to rely, when I seek to go beyond the concept A, and to know that another concept B is [a priori] connected with it?....What is here the unknown = X which gives support to the understanding when it believes that" it has synthetic a priori knowledge? Clearly this question is very important to Kant's entire philosophical programme, and also it is clearly a very difficult question.

See Synthetic statements necessary or empirical