intuition, in philosophy, way of knowing directly; immediate apprehension. The Greeks understood intuition to be the grasp of universal principles by the intelligence (nous), as distinguished from the fleeting impressions of the senses. The distinction used by the Greeks implied the superiority of intellectual intuitions over information received by the senses. Christian thinkers made a distinction between intuitive and discursive knowledge: God and angels know directly (intuitively) what men reach by reasoning. René Descartes insisted that there are not two faculties of intuition (the sensual and the intellectual) but only the faculty of intellect; sensual experience, although it appears necessary in practice, is not essential to knowledge. John Locke and others criticized Descartes's position, and under the influence of such criticism perception and the intellect came to be regarded as two separate, intuitive faculties, both necessary for genuine knowledge. Immanuel Kant took sense perception to be the paradigm of intuition, although pure intuitions of space and time were also basic to his system. For Henri Bergson, intuition was an evolved, conscious form of instinct, an unmediated experience of the external world or of the self. Bertrand Russell formulated the conceptual-perceptual distinction as the difference between "knowledge by description" and "knowledge by acquaintance" and Russell also postulated a faculty analogous to sensation that apprehended universals. The logical positivists felt it was unnecessary to posit such a faculty, and explained the apprehension of nonsensory intuitive (or noninferential) knowledge as the result of psychological conditioning in the learning of a language. To know that all events are caused is to have learned the usage of the terms event and cause. Critics have argued that such a position confuses the learning of a fact with the learning of a word. The role intuition plays in mathematics and ethics has provoked lively debate in the history of Western philosophy. According to mathematical intuitionism, mathematical knowledge rests on mathematical concepts that are immediately clear and irreducible. According to ethical intuitionism, there are fundamental ethical truths that can be known directly and do not have to be inferred.