The third form of idealism is empiricism. Locke, Berkeley, Hume and William James are the famous exponents of empiricism. In Berkelian empiricism, the objective reality of the external world is denied. Only the percipient mind is considered to exist really. It is the contention of the empiricists that the things, unless they are perceived by any mind, do not exist. It follows from this view that all the things, except those which are perceived by us, are unreal.