Aristotle's claim that "it is impossible even to think without a mental picture" (On Memory and Recollection 450a) has frequently been echoed by subsequent philosophers. David Hume equated thinking with having mental images, since he appears to have considered ideas and images to be the same; for of any sense impression "there is a copy taken by the mind, which remains after the impression ceases; and this we call an idea" (Treatise of Human Nature, Book I, Part I, Sec. ii). The sole contents of the human mind are original impressions and these copies of them. Thomas Hobbes was stating much the same view when he said, "Imagination therefore is nothing but decaying sense" (Leviathan, Ch. 2).

Many other philosophers have also accepted the existence of such mental contents without examining their nature; they had assumed that images are things whose nature or existence is obvious to all human...