Berkeley agrees with Locke that physical objects and God can only be known by experience, that the self is known by reflection, and that mathematics does not give us knowledge of real existence; he is thus an empiricist. But he rejects Locke's claim that physical objects are distinct from ideas and composed of insensible material particles. He takes this to be unempirical and meaning less on empirical grounds (since they are insensible) and even contradictory (since we cannot think of something completely independent of thought).

As we saw, Locke holds that the mind directly perceives ideas and indirectly perceives real physical objects. Since the idea is distinct from the material object and the idea's existence in the mind does not imply that the object exists, Berkeley holds that the empirical is restricted to our ideas and that matter is unknowable. A strict empiricism is inconsistent with the existence of matter and the only way to defend it, according to Berkeley, is to make the distinction between appearance and reality within the realm of ideas and not between ideas and nonideas. Berkeley calls this doctrine immaterialism, but its more usual name is idealism; Locke's theory is a version of realism. Let us look more closely at these theories.

In general, to be a realist about a range of entities is to claim that they are real and exist independently of what is thought about them. One may be a realist about universals such as redness and goodness (as Plato was) or a realist about moral facts (a moral realist).