The theme of Berkeley’s third dialogue is objections to immaterialism.
The dialogue starts off with Hylas and Philonous retracing the steps they have taken in the first two dialogues (524-26). This is handy material for reviewing Berkeley. Instead of re-reading the first two dialogues, just look at these two pages. They give Berkeley’s own summary of the argument.
The summary ends with the claim that things would exist even if we were not here to perceive them. That is because, according to Berkeley, they are ideas in God’s mind. This leads to the next major section. Hylas objects that we do not have any understanding of God’s mind. What is the advantage in saying “sensible things exist in God’s mind, in an immaterial spirit” over saying “sensible things exist independent of any minds, in matter”? Philonous, of course, responds and Hylas concedes the point (526-28).
Then there is an extended discussion of objections to immaterialism followed by Philonous’s replies (528-40). I counted twenty, although you could make a reasonable case for a few more or a few less. These end with an extended defense of the compatibility of Philonous’s position with the Biblical account of the origin of the world; that is Berkeley’s attempt to deal with a question David raised (537-40).
The third dialogue ends with Hylas conceding that the materialist is the skeptic, understood as is someone who “denies the reality of sensible things” (Berkeley [1713] 2019, 497L). The immaterialist, by contrast, is conceded to have the common sense view that we see (hear, taste, etc.) sensible objects and so we know they exist as we see (hear, taste, etc.) them. There are some additional remarks about how to think about objections that have not been considered (542) and whether it could be appropriate to use the term “matter” to refer to sensible things (542-43).
We are going to start with the second major section: Hylas’s objection that the mind of God is just as poorly understood as matter is (526-28).
Then we are going to take on a selection of the twenty objections to immaterialism. But not all of them! These seem to be the most interesting.
What is the difference between perceiving a real thing and imagining a thing? This is a question Anjali raised. See pp. 529 and 535.
If all things are in God’s mind, then doesn’t God contain all sorts of bad things? And isn’t that inconsistent with God’s being perfectly good? Also, does God feel pain? Does he feel pleasure? As a bonus, there is a position on free will tucked in there. I think this speaks to points that Evan and someone else (sorry!) raised. (529-30, 531R-532L)
If our sensations are real things, then all sorts of perceptual distortions would be real: oars in water would be crooked, the moon would be one foot tall, and so on. Furthermore, there would be no such thing as different perspectives on an object; each perspective would be a different object. So you could not use a microscope, say, to get a closer look at my skin cells. What you see in the microscope would be different from what you see with the naked eye. (530, 534-35)
How is the idea in my mind the same as the idea in your mind? When we agree “that’s a tree,” are we talking about the same thing? (535-36)
How does an unextended mind contain extended things? (536-37)