There are two quite different uses of the term idea in philosophy. The term idea is used for the denizens of Plato's heaven. Sometimes form is used as a less misleading translation of eidos. Plato's ideas or forms are not parts of our minds, but objective, unchanging, immaterial entities

that our minds somehow grasp and use for the classification of things in the changing world, which Plato held to be their pale imitations. John Locke uses the term idea for that which the mind is immediately aware of, as distinguished from the qualities or objects in the external

world the ideas are of. This use for the term leaves it rather vague. Idea can be the images involved in perception, or the constituents of thought. Hume calls the first impressions, the latter ideas, and the whole class perceptions. For Hume, the class of impressions includes passions (emotions) as well as sensations. A feeling of

anger would be an impression, as would the sensation of red brought about by looking at a fire truck.

Later memory of the feeling of anger or the fire truck would involve the ideas of anger and red.The conception of ideas as immediate objects of perception and thought, intervening between our minds and the ordinary objects we perceive and think about, was part of a philosophical

movement, sometimes called "the way of ideas," greatly influenced by Descartes's Meditations. Descartes there uses a form of the argument from illusion to motivate the distinction between the mental phenomena we are certain of and the external reality that is represented by them.