The Fork of David Hume

The principle that conclusions about what ought to be cannot be deduced from premises stating only what is, is sometimes called Hume’s Law. His employment of another distinction between fundamentally different kinds of position is now often nicknamed Hume’s Fork. It is important to realize in both cases that the point is: not that every statement actually made express some proposition falling unequivocally into one or other of this contrasting categories; but rather that we need always to be aware and to to account of the distinction upon which Hume is insisting.

Much the same applies to the Law of the Excluded Middle and to the Law of Non-contradiction. The claim that there can be no middle way between that and not-that, the contention that nothing can be at one in the same time and in the same respect both this and not-this, are to be understood more as prescriptive laws for good thinking than as descriptive laws of actual thought. Certainly, if you ask whether some vague, muddled ambiguous, or otherwise inadequate phrase describe some situation, then the only possible direct answer may be: � It does, and yet it does not. �Certainly also, people all too often do believe and assert incompatibles. Yet it is only in so far as respect is shown for consistency that any thought at all, even bad thought, can proceed: there can indeed be no language of any kind saves in so far as there are respected rules determining when its word or other elements are and are not correctly applicable. These Laws of thought, therefore, are to be both understood and cherished, partly as partly defining the essential nature of thought, and partly as describing hypothetical imperative for all those concerned for clarity and truth.

Hume published his great Treatise anonymously, and before he was thirty. He later became dissatisfied, and in the end formally disowned it. In his late thirties, when he had won a reputation with political and general essays, he produce the two inquiries over his own name. His way was to represent in an improved, more palatable form whatever materials from the treatise he still considered fit for salvage. Hume’s Fork was an instrument fully fashioned only in the first Inquiry:

All objects of human reason or inquiry 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; and, in short, every affirmation which is either intuitively or demonstratively certain. �That the square of the hypotenuse is equal to the square of two sides’, is a proposition which expresses a relation between these figures. �That three times five is equal to the half of thirty’, expresses a relation between these numbers. Propositions of this kind are discoverable by the mere operation of thought, without dependence on what is anywhere existent in the Universe. Though there