Hume's Fork

Two very famous David Hume quotes illustrate what has come to be known as “Hume’s fork.” This is an either-or situation regarding only two cognitive possibilities; viz., truths of reason and truths of “fact.” Note that, for Hume, any language that is neither a truth of reason or a truth of fact is to be “cast into the flames.”

Hume writes: "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

And Hume again: "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

What shall we think of this “fork?” One way to understand it is as a precursor of Logical Positivism (exemplified by, e.g., the early 20th-century philosopher A. J. Ayer). Logical Positivism has been sufficiently refuted as follows.

1. Call Hume’s words from the Treatise P1 (Premise 1).


2. Truth is confined to either “truths of reason” or “matters of fact.”


3. If a putative statement is neither of these, then it’s only good for starting a fire; i.e., it is cognitively worthless.


4. But P1 is itself neither a “truth of reason” nor a “matter of fact.”


5. Therefore P1 is worthless and to be tossed into the flames.

Hume’s fork is thus rendered, by its own truth test, to be itself not true and to be tossed into the flames. In the same way philosophers such as the later Wittgenstein discarded Ayerian Logical Positivism's "verification principle" in the early twentieth century.