Hume's fork

Hume's fork is an explanation, developed by later philosophers, of David Hume's aggressive, 1730s division of "relations of ideas" from "matters of fact and real existence". On the necessary versus contingent (concerning reality), the a priori versus a posteriori (concerning knowledge), and the analytic versus synthetic (concerning language), truths relating ideas (abstract) all align on one side (necessary, a priori, analytic), whereas truths on actualities (concrete) always align on the other side (contingent, a posteriori, 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, and therefore a tautology—necessarily true by logic but uninformative on the world's state—whereas the synthetic is true by its terms' meanings in relation to a state of facts, contingent.

Hume's strong empiricism, as through Hume's fork as well as Hume's problem of induction, was taken as a threat to Newton's theory of motion. Immanuel Kant responded with rationalism in his 1781 Critique of Pure Reason, where Kant attributed to the mind a causal role in sensory experience by the mind's aligning the environmental input by arranging those sense data into the experience of space and time. Kant thus reasoned existence of the synthetic a priori—combining meanings of terms with states of facts, yet known true without experience of the particular instance—crossing the tongs of Hume's fork and thus saving Newton's law of universal gravitation.

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