Philonous now launches into his full idealist account. All there is in the world, he tells Hylas, are ideas and the minds that perceive or conceive them (called "spirits"). Some of our ideas are "real things" and some are not. For instance, products of our imagination are not real chairs, tables, etc., they are simply imagined chairs, tables, etc. Ideas in our memory are also not real things, but mere ideas. What are real things are our sensations. Or, rather, real objects are collections of sensations. When I see a chair, for example, my sensations of brown, hard, a certain size and a certain shape together actually comprise the chair. The chair just is that collection of sensations. The way that we can distinguish real things from mere fictional ideas, then, is the same way in which we can distinguish our sensations from all other ideas: real things are more vivid and they come to us involuntarily.
Because our sensations come to us involuntarily, we can conclude that we do not cause them. If they depended on our will, then we would be able to control when and how we had them. We can reason from this fact that there must be some other being controlling which sensations we have. This being is God, the ultimate, infinite perceiver. God plays a central role on this view: what makes an object exist, is not whether I perceive it or you perceive it. What makes an object exist is simply whether God perceives it. God brings all things into existence by conceiving of them, and maintains them in existence by continuing to conceive of them. All things exist in God's mind. Periodically, God will allow us access to these ideas, in certain fixed patterns which we call "the laws of nature". We experience these ideas as sensations. For instance, whenever God gives us the sensation "seeing fire" he accompanies it with the sensation "feeling heat", whenever he gives us the sensation "touching cat" he accompanies it with the sensation "feeling softness", and so on. "Cat", "soft", "fire", and "heat", though, are all simply ideas in God's and our own mind; none of these things has a mind-independent existence out in the world. In fact, there is no such thing as "out in the world". There is nothing outside of minds, except for minds themselves (or spirits) and all finite spirits (i.e. human beings) exist within God's mind.
After Philonous lays out this picture of the world, Hylas asks how this view is different from Malebranche's view. The Cartesian philosopher Malebranche, claimed that we see all things in God, a view which, on its surface, sounds much like Philonous' idealism. Malebranche's theory, though, goes like this: since the soul is immaterial, it is incapable of perceiving material things. The soul, therefore, joins with God who is Himself an immaterial, spiritual substance, and through this union, gains access to all ideas and the logical connections between them. This view, as Philonous is quick to point out, is a far cry from real idealism. Malebranche posits the existence of mind-independent material objects: they exist, we just do not have any immediate access to them. So not only does he run into all the usual materialist problems, he has an even worse problem to contend with: his material world is completely useless.
Hylas could have avoided skepticism by relying on Locke's use of inference to the best explanation — the best explanation for so many marks of our sensory experience being that this experience is caused by mind-independent material objects that resemble our sensory ideas. However, he chooses not to do this, and now he is stuck trying to avoid skepticism by embracing idealism. The rest of the book is an attempt to flesh out the idealist picture, and to show why it is the best account of reality.
According to Berkeley's idealist view, there are only three types of things: there are ideas, there are finite minds, and there is God. We will treat the role of ideas more fully as Philonous gives more details of his theory, and we will also have more to say about spirits later on. But it is important to settle the issue of God immediately, because God is really the grounding for the entire metaphysical system, and His role is often misunderstood.
The way that Berkeley proves the existence of God is to ask two questions. First, since my sensations are not caused by me, who causes them? And second, how do objects stay in existence when I close my eyes? The answer to these questions is that God must keep everything in existence, and cause our sensations. But how? There have been two different positions that are often attributed to Berkeley on this point. On the first of these, God's role is as the absolute perceiver. Things have a continued existence (instead of constantly flickering in and out of existence whenever I close my eyes) because God is always perceiving them. There is a lot of textual evidence for this reading of Berkeley; look, for instance, at 2.212 and at 3.230–1. But there is also something very unappealing about this view: we retreat to God only when we need him. His role is important, but it is not central in the way that Berkeley seemed to promise it would be. Conceiving of God as back up perceiver is no better than Locke's conception of God as the link between the physical world and the mental, or Descartes' conception of God as the guarantor for the truth of our clear and distinct perceptions; in all of these cases God is more of a convenient fill-in, a Deux es Machina, than a grounding for the metaphysical system.
The other reading gives God a much more central role in the system. God is not the fill-in perceiver in this view, rather it is God's perceptions of things in virtue of which they are originally said to exist. All ideas (and spirits too, Berkeley seems to suggest) exist in God's mind, and we only gain access to these when He chooses to reveal them to us. It is only God, then, that is external to finite minds. There is no world outside of God.
Given this reading of Berkeley it is easy enough to see why he thought that his idealism was a powerful antidote to atheism. Anyone who accepted this view of the world would need to accept God as a central part of it. God cannot easily be removed from this system, as he could be from Locke's or Descartes' systems; remove God from the system and there is no system. The system boils down almost exclusively to this: objects are ideas in the mind of God; objects exist because God perceives them. How could we replace God in such a system? What could be responsible for keeping all our ideas in existence and causing our sensations, if not God? It is for this reason that all later idealist systems were also highly spiritualized, even if not religious in a traditional sense. Since material objects cannot play this role, and we ourselves certainly cannot play this role, that leaves only some spiritual being, greater and more powerful than ourselves. In this, at least, Berkeley was absolutely right: one cannot be an idealist and an atheist at the same time; to believe in idealism, is to believe in a spiritual being who grounds the entire world.
There is also another related position that has been frequently attributed to Berkeley: phenomenalism. According to this interpretation of Berkeley's thought, the answer to the question, "how do things stay in existence when I close my eyes?" has nothing to do with God. Instead, goes this line of reasoning, things stay in existence because their existence is not to be actually perceived, but only to be possibly perceived. A thing, on this phenomenalist understanding, is a permanent possibility of sensations. To say, for instance, that there is a table in the other room, is just to say that if someone were to go into the other room, they would have sensations of a table. Both Mill and Russell actually ascribed to a view like this one, which rejects the claim that esse is percipi and says instead that being is to be perceivable. There is some textual evidence that suggests that Berkeley ascribed to this view as well. For instance, at the end of the third dialogue, Philonous says that when the Bible speaks about God creating the world, what it means is that God set things up so that if there were perceivers around they would have had such and such sensations.
There are many difficulties that arise from this view. In order to fulfill the antecedent of the conditional (the "if " part), you need some mind- independent understanding of what the conditions are. For instance, in order to make the above claim about the table, you would have to believe that there really is a room you could walk into. Perhaps, though, we could understand the antecedent as follows: If I were having standing in the other room sensations then I would have table sensations. But still another worry remains: what grounds these conditions? What makes it the case that if I had standing in the other room sensations I would have table sensations? We want to say, of course, that what grounds these conditions are some facts about the world: the fact, for instance, that there is a table in the other room. But Berkeley, as an idealist, certainly cannot say this.
Berkeley probably does ascribe to something like this phenomenalist understanding of existence, in addition to his plain idealist understanding, but his phenomenalism can only be understood if we add in the central role that God must play in his system. It is God, on his view, who grounds all conditionals: it is because of God that if I had standing in the other room sensations, I would have seeing table sensations. These ideas maintain this relation to one another because God chose to place these ideas in this relation to one another; they go together constantly in His perception, and so also go together constantly in our perception.