Hume's Fork

Hume's fork is an explanation, developed by later philosophers, of David Hume's aggressive division, introduced in the 1730s, between "relations of ideas" versus "matters of fact and real existence".

By Hume's fork, relations among ideas are strictly divided from states of actuality. Thus, among the necessary versus contingent (concerning reality), the a priori versus a posteriori (concerning knowledge), and the analytic versus synthetic (concerning statement), relations among ideas or the abstract are necessary, a priori, and analytic, whereas actualities or the concrete are contingent, a posteriori, and synthetic.

(The necessary is a state true in all possible worlds, usually by mere logical validity, whereas the contingent hinges on the way the particular world is. The a priori is knowable before or without, whereas the a posteriori is knowable only after or through, experience in the area of interest. The analytic is a statement true by virtue of its terms' meanings, but tends to be a tautology—necessarily true by logic but uninformative as to the state of the world—whereas the synthetic is true by its terms' meanings in relation to a state of facts, contingent.)