Idea
Collier's New Encyclopedia (1921)
a mental image, form, or representation of anything. The word idea has been taken in very many and very different senses, the history of which would be a history of philosophy. Aristotle (384-322 B. C.) taught that though the One, apart from and beside the many, does not exist, none the less must a unity be assumed as (objectively) present in the many; and the Stoics (Zeno, 355-263 b. c.) maintained the doctrines of subjective concepts formed through abstraction. According to Plutarch of Chæronea (toward the end of the 1st century), the ideas were intermediate between God and the world; they were the pattern and God the efficient cause. For Plotinus (203-270) the primordial essence was elevated above the Platonic ideas, which were emanations from the One. St. Thomas of Aquin (1227-1274) recognizes a form in which the universal exists before things-viz., as ideas in the divine mind. For Descartes (1596-1650), "ideas are the forms of things received into the soul"; for Spinoza (1632-1677) the "concepts epts formed by the mind as a thinking thing"; and Locke (1632-1704) says, "whatsoever the mind perceives in itself, or is the immediate object of perception, thought, or understanding, that I call an idea." In the philosophy of Leibnitz (1646-1716) ideas are the active forces of his monads; Berkeley (1684-1753) used the word as equivalent to phenomenon; Hume (1711-1776) defines ideas as "copies of perceptions," and Condillac (1715-1780) as "mental representations of objects of apprehension." Kant (1724-1804) gives the name of ideas to those "necessary conceptions of the reason for which no corresponding real objects can be given in the sphere of the senses." A According to Erasmus Darwin (1731-1802), an idea is a "contraction, or motion, or configuration of the fibers which constitute the immediate organs of sense." James Mill (1773-1836) calls ideas "what remain after sensation has gone," and Herbart (1776-1841) "typiof the Commission to establish civil gov-cal conceptions." Schopenhauer (1788- 1860) posits as intermediate between the universal will and the individual in which it appears, various ideas as real species forming stages in the objectification of the will.
What Has Changed in the Last Century
Collier's in 1921 could still treat "idea" as a settled starting point — "a mental image, form, or representation of anything" — and trace it through a roll-call of philosophers ending with Schopenhauer. Within a generation that starting point was under direct attack: behaviourist psychology dismissed introspected inner images as unusable evidence, and Wittgenstein's Philosophical Investigations (1953) and Gilbert Ryle's The Concept of Mind (1949) argued that treating ideas as private objects inspected by the mind was a confusion of grammar. The word did not disappear so much as get replaced: from the 1950s the cognitive sciences rebuilt the same territory as "mental representation," studied with reaction-time experiments and, since the 1990s, brain imaging rather than the armchair. One concrete gain the older list could not have anticipated is that variation in mental imagery is now measured, including the inability to form images at all, named aphantasia by Adam Zeman's group in 2015.
Written for this edition, 2026.