These are entities that exist only as contents of some mind. Ideas in this sense should be distinguished from Plato's Ideas or Forms , which are non-physical but exist apart from any conscious beings. The image of a Platonic Form that occurs in a person's mind would be an idea in our sense. Beginning in the seventeenth century all objects of consciousness were held to be ideas. For example, we are conscious of ideas when we imagine, remember, dream, or think about some concept or proposition.
Ideas are subjective in that individuals can be aware only of their own ideas.
If two individuals are imagining Pegasus or thinking about the Pythagorean theorem each is directly aware of a distinct idea, although these ideas may share many features. This is analogous to the sense in which two reproductions of the Mona Lisa are distinct objects even though most of their properties are identical, but it is impossible for one individual to inspect another's ideas. Reflection on the nature of our perceptual experience led Descartes and Locke, among others, to argue that even when we are perceiving we are directly aware of ideas, not physical objects. For example, touching a hot object or walking into a wall may cause someone to feel pain. The pain is caused by a physical interaction between the object and the perceiver's body, but the pain exists only as long as the individual is conscious of it.