This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Hume divides everything the mind can know into two, and only two, kinds, a division so clean it has come to be called Hume's fork. Relations of ideas are the truths of mathematics and pure logic, discoverable by thought alone, certain, and true independent of anything the world happens to contain, the way three times five equals half of thirty is true whether or not anyone ever counts anything. Matters of fact are everything else, every claim about how the world actually is, and none of them, Hume insists, can be known with that same certainty, because the opposite of any matter of fact remains conceivable without contradiction; the sun's failing to rise tomorrow is perfectly imaginable, unlike a triangle with four sides. The fork is Empiricism carried to its most severe conclusion. If a claim is neither a relation of ideas, provable by pure reason, nor a matter of fact, grounded ultimately in some sensory impression, Hume's famous instruction is blunt: commit it then to the flames, for it can contain nothing but sophistry and illusion. This is the standard he turns on causation itself, arguing that the necessary connection everyone assumes links cause to effect is never actually observed, only the constant conjunction of one event following another, with necessity itself supplied by habit rather than perceived in the world. Locke had trusted experience to deliver real, substantive knowledge of things; Hume, starting from the same empiricist premises, arrived somewhere far more austere, and considerably harder to live with once believed.