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.
David Hume (1711–1776), Scottish philosopher. Enquiries Concerning the Human Understanding and Concerning the Principles of Morals, sect. 4 ("Sceptical Doubts Concerning the Operations of the Understanding"), part 1, p. 25, ed. L. Selby-Bigge, M.A., 2nd edition, London, Oxford University Press (1902).