After a person has sensations, his mind forms ideas. For instance, you see a red rose that smells sweet and soon are able to form ideas of redness and sweetness. Obviously, you don't need to be looking at a red thing or presently smelling a sweet thing to have the ideas of such things in your mind. Anything of which the mind is aware Locke calls an “idea.”

How can ideas be categorized? Locke distinguishes between simple ideas, which come from one sense, such as bitter, sour, cold, and hot, which contain no other ideas and which cannot be created by you, and complex ideas, which are produced by the mind when it compounds and combines simple ideas. God is one example of a complex idea. To arrive at the idea of God, you can simply enlarge your stock of simple ideas such as existence, time, knowledge, power, goodness, and so on. Complex ideas may also be strange things such as unicorns or satyrs that have no actual existence but will always be analyzable into a medley of simple ideas acquired through experience.