Three Dialogues between Hylas and Philonous

Here Philonous presents his famous claim that, when it comes to sensible objects (i.e. objects we can immediately experience through our sensory faculties), esse is percipi — that is, their existence is to be perceived. Such objects do not have any mind-independent existence. They exist if they are perceived, and they do not exist if they are not perceived.

One of Berkeley's arguments for the claim that esse is percipi is his famous Master Argument. This argument seeks to establish that it is impossible to conceive of an unconceived object, because the moment you try to conceive of the object, it is conceived (by you). This argument was wildly influential in the history of philosophy, inspiring such thinkers as Kant, Hegel, and Russel.

One of Berkeley's primary aims is to show that materialism leads to skepticism, while his own idealism does not. Because materialists believe that the only sort of real existence is an absolute existence independent of all minds, once it is shown that sensible objects have no such an existence (as Berkeley thinks he has conclusively demonstrated) the materialist is reduced to admitting that sensible objects have no real existence. Berkeley, on the other hand, maintains that sensible objects do have a real existence, but that it is a mind-dependent existence.

In response to Hylas' charge that his idealism is in flagrant violation of common sense, Philonous tries to defend himself by pointing out that, contrary to Hylas' accusation, his view is actually the same as the view of the common man. Both he and the common man agree that the best way to know that something exists is to experience it — see it, hear it, smell it, taste it, and you cannot doubt that it is there.

This is a clear statement of Berkeley's idealism. He is not trying to thin down reality, he tells us, but rather to bulk up ideas. His aim is not to prove that the world is composed of mere ideas, mere figments; rather, he is saying that the world is comprised of these very substantial, very real things, that happen to be mind-dependent. These are as real as we now mistakenly think that matter is. The only difference is that we believe that matter can exist outside of any mind, whereas he happens to know that real things cannot exist independently of mind.