Philonous now resumes the task of demonstrating that we do not mediately perceive mind-independent material objects — that is, that we have no reason to infer their existence from our immediate experience of the world. He has already demonstrated, he believes, that we do not infer matter as a substratum, or support for observable qualities, and now he will try to show that we cannot infer material objects as the archetypes for our ideas. In the second dialogue he will consider the third possibility for an inference to mind- independent material objects: material objects as the cause for our ideas.
Hylas, of course, is the one who introduces the idea of material objects as the archetypes for our ideas. His hypothesis is familiar enough: our ideas are copies of things out in the world; they resemble objects out in the world and so when we perceive our ideas we also gain access to the material objects which they resemble. Ideas are related to material objects, in other words, like a photograph is related to the person photographed. Our immediate access is to the photograph, but through it we gain access to the person herself. In the case of ideas, it would go like this: there is a tree out in the world, and then there is an idea of a tree, which is just a copy of that tree out in the world (like a snapshot). What we perceive is the copy, but through that copy we come to know about the original tree as well.
In order to debunk this notion of material objects as the archetypes for our ideas, Philonous presses of the notion of representation. He says that it is crazy to think that an idea can represent a mind-independent material object. Only an idea, he tells Hylas, can resemble an idea. Take a materially existing tree and the idea of a tree. How can these two things resemble one another? The idea of the tree is green and brown. But the material tree cannot be green and brown because colors, he has already shown, cannot exist outside of the mind. The idea is large and tree-shaped. But the material tree cannot have either of these properties either, because they, too, are mind-dependent. In short, our ideas of things (trees, flowers, chairs, etc.) are completely comprised of sensible qualities and no sensible qualities can exist outside of the mind. So whatever exists outside of the mind, cannot in any way resemble our ideas. Ideas are sensible, and real things are not. To say they resemble one another would be like saying that something invisible resembles something colored. In addition, our ideas are fleeting and variable, while material objects are supposed to be stable and constant. In this way, too, they cannot resemble one another.
Hylas is now finally reduced to skepticism. He admits that no sensible things exist outside of the mind, and concludes from there that no sensible things have any real existence. Denial of the real existence of sensible things, remember, was precisely our definition of skepticism. So Hylas' materialism really has led him straight into skepticism. In the next dialogue, Philonous will demonstrate how idealism can pull him out of this skepticism.
Since all of us are probably just as reluctant as Hylas to accept Berkeley's idealist picture, it is worthwhile to analyze how we could be materialists and still avoid skepticism. Is Berkeley really right in claiming that materialism leads us straight into those pervasive doubts? Other philosophers have effectively avoided this snare. In particular, we can look at how Rene Descartes and John Locke avoided skepticism. Both of them had to deal with these worries because both of them ascribed to a mediated theory of perception (i.e. a theory of perception on which our immediate access is to ideas, not objects).
Both Descartes and Locke agree that we must infer the existence of mind- independent material objects; proof of their existence is not given to us immediately in experience. Descartes infers the existence of mind-independent material objects from his innate ideas. In particular, he uses his innate idea of God as a perfect being to establish that God could not be a deceiver, and, from there, reasons that if our sensations were caused by anything other than mind-independent material objects God would be deceiving us. We will see later that Philonous actually considers a similar line of reasoning, and then rejects it.