A Treatise Concerning the Principles of Human Knowledge

A short treatise arguing that nothing exists outside minds and their ideas. The Introduction attacks Locke's doctrine of abstract general ideas, which Berkeley blames for much of the confusion in philosophy, and warns that words often mislead us. Part I then argues that the objects of sense are ideas, and that for such things to be is to be perceived (esse est percipi). Since an idea can only resemble another idea, and since primary qualities such as shape depend on the mind as much as colour does, the notion of unthinking material substance is either empty or contradictory. What exists are active spirits, which perceive, and passive ideas, which are perceived. The regular order of our sensations, which we call the laws of nature, is caused directly by God. Berkeley then answers objections: that his view abolishes real things, that objects would vanish when unobserved, and that it conflicts with science and Scripture. He presents the result as a cure for scepticism and atheism.

It is the clearest case for idealism in English, and arguing with it is still one of the best ways to work out what you mean by 'matter', 'perception' and 'real'.