philosophical theory of knowledge based on the assertion that the mind perceives only mental images (representations) of material objects outside the mind, not the objects themselves. The validity of human knowledge is thus called into question because of the need to show that such images accurately correspond to the external objects. The doctrine, still current in certain philosophical circles, has roots in 17th-century Cartesianism, in the 18th-century empiricism of John Locke and David Hume, and in the idealism of Immanuel Kant.