Enquiry Concerning Human Understanding - pg. 108

SECTION 4
SCEPTICAL DOUBTS CONCERNING
THE OPERATIONS OF THE
UNDERSTANDING

PART 1

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 ate the sciences of Geometry, Algebra, and Arithmetic; and in short, every affirmation, which is either intuitively or demonstratively certain. That the square of the hypotenues 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 any where existent in the universe. Though there never were a circle or triangle in nature, the truths, demonstrated by EUCLID, would for ever retain their certainty and evidence.