Berkeley's aim in the first dialogue is to prove that materialism is false — that is, that we have no reason to believe in the existence of mind-independent material objects. With this end in mind, he launches a two- staged attack against the thesis . He attempts to prove first that we have no immediate perception of mind- independent material objects, and then that we have no basis on which to infer the existence of mind- independent material objects from our immediate experience. Since Berkeley is an empiricist, establishing that we do not obtain evidence for the existence of mind-independent material objects in either of these two ways, amounts in his eyes to proving that we do not obtain evidence for mind-independent material objects, period. For an empiricist, all knowledge must either come directly through sensory experience or else be inferred on the basis of such experience.
In order to prove that everything we perceive in our immediate experience is mind-dependent, Berkeley presents us with two arguments. Grounding both of these arguments is the seemingly unobjectionable claim that what we immediately perceive of the world i s sensible qualities (such as color, taste, smell, heat, shape, size, and so on). In the first of these arguments, he tries to get us to admit that our experience of the world (at least our experience of color, taste, sound, heat, and smell, as opposed to size, shape, and motion) fundamentally involves pleasure and pain, and that these sensations cannot exist in material objects. By hooking up all of our sensations of secondary qualities with pleasure and pain, then, he forces us to admit that none of these sensible qualities can exist outside of mind. Berkeley uses the following line of reasoning. Imagine that you are experiencing intense heat, he instructs us. How do you experience this? As pain, naturally. But can pain exist in an unsentient objec t? Of course not. So pain cannot be in material objects; pain can only be in a mind. But if we feel intense heat as pain, that means that intense heat also cannot exist outside of mind. So intense heat is mind-dependent. This means that all heat must be mind-dependent, since intense heat is obviously the same kind of thing as all other degrees of heat.
The argument from pleasure and pain only applies to secondary qualities, but Berkeley must also prove that primary qualities are mind dependent if he is to prove that everything we receive through immediate experience is mind-dependent. His second argumen t, therefore, applies to primary qualities as well. Here Berkeley points to instances of perceptual relativity, such as the fact that colors can look different in varied lighting conditions or that a piece of wheat can be big to a mite and small to a huma n. Given that we have these highly variable experiences of both primary and secondary qualities, Berkeley concludes that what we are experiencing cannot be anything mind-independent. After all, material objects are supposed to be stable things, and if the y are not changing constantly, then they cannot be what we are experiencing as changing so frequently. With these two arguments, Berkeley feels he has shown that everything we immediately perceive (i.e. all sensible qualities) is mind-dependent. In other words, he thinks he has shown that we get no evidence of mind-independent material objects from our imm ediate experience. If we do have any evidence for the existence of mind-independent material objects then, this must come from some sort of inference that we make based on our immediate experience. Berkeley's next task, therefore, is to show that no such inference is warranted. He shows, first, that we cannot infer the existence of matter as some sort of support for sensible qualities (i.e. as a substratum), because this notion is incoherent; next he shows that the idea of material objects as the arch etypes for our ideas is equally incoherent. He does the same for the thesis that material objects are the cause of our ideas. Since he believes that these are the only three inferences one might make, once he has shown that these are unwarranted, he belie ves that he has conclusively shown that we have no evidence at all for the existence of mind-independent material objects.
In addition to the arguments outlined above, Berkeley also has another argument which he thinks proves conclusively that materialism is false. This argument seeks to demonstrate that it is actually inconceivable for an object to exist outside of a mind. T he argument goes like this: (1) We can conceive of a tree existing independent of an out of all minds whatsoever only if we can conceive of the tree existing unconceived. (2) But an unconceived conceived thing is a contradiction. (3) Hence we cannot conce ive of a tree (or anything else) existing independent and out of all minds.
In plainer terms: In order to conceive it possible for a tree to exist outside of all minds, we need to be able to think of an unconceived tree. But as soon as we try to conceive of this unconceived tree, we have conceived it. So we have failed. Try to im agine, for instance, a tree deep in some primeval forest. Surely this tree has never been conceived. But it just was. As soon as you imagined it, it was conceived.
It is agreed by pretty much everyone, that this argument is terrible. But people disagree about exactly what mistake Berkeley is making here. Most people think that Berkeley's problem is that he fails to distinguish between the act of perception and the c ontent of the perception. It is one thing for me to be imagining the tree, and quite another for the content of my imagining to be of an unconceived, unperceived tree. According to this analysis, Berkeley is right to point out that it is a contradi ction to assert that there is some X that is both unconceived and is also conceived by me. However, he is overlooking the fact that the content of the conception can be isolated from the act of conception. I can have a conception, the content of which is: unconceived tree.
Berkeley thinks that there are only two sorts of things in the world: ideas and the minds that have them. This does not mean, however, that he thinks that all of reality is just a subjective figment of our imaginations. He believes firmly in the existence of a "real world". He just thinks that this real world is made up entirely of ideas.
In particular, real things are collections of sensations. We can distinguish real things from our other ideas (such as the products of our imagination and memory) because they are more vivid, and they are involuntary. In other words, we can tell which ide as are real things by telling which of our ideas are sensory perceptions.
Berkeley's theory that real things are just collections of sensations is often summed up by the Latin phrase, "Esse is percipi", which means, "their existence is to be perceived". This is just another way of saying that real things are mind-dependent, or that they are ideas.
By making real things into ideas, Berkeley thinks that he has posited a world system that is immune to skepticism. Skepticism infiltrates a system in two ways: it raises doubts about whether anything really exists, and it raises doubts about whether appea rances adhere to reality. Neither of these doubts can be raised on Berkeley's picture. Since real things just are sensations, once you have a sensation of, say, a tree, it is a contradiction to doubt that the tree exists. The tree just is the sensation, a nd you cannot doubt that you had that.
In addition, there is no room for wondering whether the tree is really like it appears to you. There is nothing to the tree other than your sensation of it. There can be, therefore, no distinction between appearance and reality.
Skepticism was one of Berkeley's main targets in the Dialogues; the other was atheism. In order to combat the first of these evil forces, Berkeley conflated appearances and reality by making ideas into "real things"; in order to combat the second, he set God up in a crucial central role, controlling and maintaining the whole idealist system.
Though Berkeley thinks that sensible objects are mind-dependent, he does not think that they depend for their existence on his mind or your mind or any human mind. Instead, he thinks that they all depend for their existence on God's mind. God brings thing s into existence by conceiving of them, and maintains their existence by continuing to conceive of them. God is the ultimate perceiver. From time to time God also allows us to perceive these ideas, in certain fixed patterns which we call "laws of nature". For instance, whenever He allows us to have the sensation "seeing fire", he accompanies it with the sensation "feeling heat". He does the same with the sensations "seeing snow" and "feeling cold", and so on.
Berkeley is certain that God must be the cause of all of our sensations, because he notices that these sensations are involuntary. While he can choose to conjure up an image of a watermelon in his imagination, he cannot just choose to see a watermelon wit h his eyes. There is something else out there which causes him to either see a watermelon or not see a watermelon, regardless of his own will. While most people would say that what is out there determining his sensation is a mind-independent material obje ct (i.e. a material watermelon), Berkeley knows this cannot be the case because he has already shown that there are no such things as mind-independent material objects (or, at least, that there is no reason to believe there are such things). Instead, he c oncludes that it is God who is causing his sensation. God, he reasons, must contain all ideas inside of Him and allow us to have access to them now and then, in certain patterns.
Positing God as the ultimate backup perceiver means that real objects do not flicker in and out of existence depending on whether any human being is perceiving them. A tree deep in an uninhabited forest exists just as truly and constantly as a tree in a s uburban park. For a real object to exist, it must simply be perceived by God. Many people have misread Berkeley on this issue, and, therefore, attribute to him a much less sophisticated view than the one he really held.