A brief outline of the first of Berkeley's Three Dialogues between Hylas and Philonous

The characters

Philonous represents Berkeley's own point of view. Hylas represents various points of view opposed to Berkeley, whether they be ordinary folks who have been deluded by philosophers, or philosophers who have rejected Berkeley's theory.

1. Hylas accuses Philonous of being a skeptic. A skeptic is someone who believes there is no truth, or that we can never know the truth. Here "skeptic" seems to mean someone who believes that nothing is real.

Hylas makes this accusation because Philonous declares that there is no such thing as "material substance", or matter. Hylas assumes that only material things are real. Therefore, someone (like Philonous) who doesn't believe in matter, believes that nothing is real, that is, denies the real existence of sensible things (the objects we perceive with our senses).

Philonous, in turn, accuses Hylas of being a skeptic. Philonous believes that there is no matter. Therefore someone (such as Hylas) who believes that only matter is real, believes that nothing is real.

Philonous: There is no matter.
Hylas: Only matter is real.
Anyone who believes both these statements is a skeptic. Since each character thinks his statement is obviously true, and sees that the other character believes the other statement, each character concludes that the other is a skeptic.

So the dialogue gets underway with Philonous trying to prove the truth of his claim that there is no matter. If he does so, then Hylas will either have to give up his own claim that only matter is real, or have to admit that he is a skeptic.

2. Before getting to the question of whether sensible things exist, the two have to clarify what they mean by "sensible things". Philonous gets Hylas to agree that "sensible things" are the things that we perceive immediately, i.e. the sensations of color, shape, sound, etc.

3. Philonous then asks whether something has to be perceived to exist. Hylas answers, "To exist is one thing, and to be perceived is another." That is, real things exist independently of the mind or of anyone's perceiving them.

4. Philonous asks, does heat really exist, then? Hylas says yes. Philonous proceeds to use the same arguments that Locke used to show that heat cannot exist in a fire, or warmth and cold in water, independent of the mind.

5. Philonous leads Hylas through the same sorts of arguments with respect to taste, smell, sound, and color. In each case he gets Hylas to admit that these are sensations that could exist only in the mind and could not belong to anything outside the mind.

6. The big turning point in the first dialogue comes when Hylas comes around and finally accepts Locke's position on primary and secondary qualities.

7. Now Philonous, who has been arguing on behalf of Locke all along, turns the tables on him. Beginning with size, he argues that all the sensations which Locke maintained resemble qualities in the material objects, i.e. the sensations "caused by primary qualities", are exactly the same as those "caused by secondary qualities" (color, etc.): they are nothing more than sensations that exist only in the mind and bear no resemblance to anything existing outside the mind.

(Note that Berkeley rejects the whole notion of "qualities", whether primary or secondary, since qualities are supposed to belong to material objects existing outside the mind, and are supposed to cause sensations in the mind. For Berkeley, objects don't cause sensations -- they are sensations.)

8. Hylas finally acknowledges that all sensible things are only sensations in the mind. But he stubbornly clings to the notion of a "material substratum", an unperceived substance in which qualities such as size and shape must reside. Still, he finds himself unable to describe this substance -- anything he can say about it will always be in terms of some sensation or other, which cannot exist outside the mind. Philonous mocks his insistence that sensible things are real only if they are made of this mysterious, unthinkable substance: "When you conceive the real existence of qualities, you...conceive something which you cannot conceive?"

9. Hylas acknowledges that he can't directly perceive matter: by definition, matter exists outside the mind, while all that we can directly perceive are sensations in the mind. But he claims that he can indirectly perceive it, just as when he sees a picture of Julius Caesar, he directly perceives lines and colors, but he indirectly perceives the long-dead Roman emperor. Philonous responds that Hylas doesn't perceive Julius Caesar at all. Someone who's never heard of Julius Caesar will perceive exactly the same picture as Hylas. Julius Caesar is not a sensible thing, but rather an idea in Hylas' mind that arises from memory (what he learned in history class) and reasoning.

10. Hylas suggests that our sensations could be copies of the material things. Philonous responds with three points:

(a) How could our sensations of an object, which are constantly changing, be copies of an unchanging material object?

(b) If one of those sensations is the true copy, how can we determine which one is the true one?

(c) How can an idea be a copy of anything other than another idea? How could something perceptible (a sensation) be a copy of something that is in principle imperceptible (a material object)? How could something visible be a copy of something invisible?

11. Hylas is on the verge of admitting that sensible things are not real, that is, that he is a skeptic, when he is saved by the bell for prayers. He'll have to deal with that issue tomorrow.