In philosophy Hume's fork is a distinction, due to David Hume, between two different areas of human study:
- All the objects of human reason or enquiry may naturally be divided into two kinds, to wit, Relations of Ideas, and Matters of fact. Of the first kind are the sciences of Geometry, Algebra, and Arithmetic ... [which are] discoverable by the mere operation of thought ... Matters of fact, which are the second object of human reason, are not ascertained in the same manner; nor is our evidence of their truth, however great, of a like nature with the foregoing.
- - Treatise of Human Nature
- - Treatise of Human Nature
- Statements about ideas - these are analytic, necessary statements that are knowable a priori.
- Statements about the world - these are synthetic, contingent, and knowable a posteriori.
Crucially, Hume notes that statements of the second type can never be entirely certain, due to the fallibility of our senses, the possibility of deception (see e.g. the modern brain-in-a-vat theory) and other arguments made by philosophical skeptics. It is always logically possible that any given statement about the world is false (note that statements like "either the Earth has precisely one moon, or not" are really truths of logic, and say nothing about the world).
Hume famously rejected the idea of any meaningful statement that did not fall into this schema, saying:
- If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: for it can contain nothing but sophistry and illusion.
- - Enquiry Concerning Human Understanding
- - Enquiry Concerning Human Understanding