The Dialogues begin with an anecdote. It is early morning, on a university campus, and our two protagonists, Philonous and Hylas, have just run into each other while each taking a solitary stroll. Philonous is pleasantly surprised to find his friend awake so early, but Hylas seems distracted and mildly agitated. He explains that he has been mulling over the assortment of insane beliefs that philosophers hold — both those who "pretend to believe nothing at all" (i.e. the skeptics) and those who "believe the most extravagant things in the world". Hylas is disturbed by the prevalence of these insane beliefs for a very practical reason: he is afraid that when common people hear supposedly learned scholars spouting off about how they know nothing at all, or else making claims that are entirely contrary to common sense, they themselves will end up becoming suspicious of the most important, sacred truths which until then they had considered unquestionable. In other words, following the lead of the philosophers, they will begin to doubt their own religious convictions and other common sense opinions.
Philonous is sympathetic to this line of thought, and confides that he himself has given up many of the views he learned in school in order to embrace common sense opinions. Hylas lets out a sigh of relief; as it turns out, he had had Philonous' own views in mind when he was worrying about crazy notions. He is extremely happy to hear that Philonous does not actually hold the wild view ascribed to him by some of their colleagues: namely, that there is no such thing as mind-independent material objects in the world, only ideas and the minds that have them.
No, Philonous corrects, he still holds that view. Hylas is now beside himself with confusion: then how can Philonous be claiming allegiance to common sense and decrying extravagant metaphysical notions? Because, Philonous explains calmly, nothing is more commonsensical than his view, as he will now demonstrate. Philonous spends the rest of the Dialogues making the case that his idealist view is the most commonsensical view in the world. His goal is to prove that, not only is his theory simpler and better supported by the evidence, but it is even immune to skeptical worries and atheistic challenges; the materialism which Hylas ascribes to, on the other hand, is incoherent and leads to straight into skepticism (and possibly even atheism).
Before launching into his elaborate argument, though, Philonous feels that he needs to establish exactly what is meant by calling someone a "skeptic". Otherwise he might be wantonly accused of skepticism just because he happens not to believe in a physical reality. A skeptic, Philonous and Hylas agree, is "one who denies the reality of sensible things, or professes of the greatest ignorance of them" (sensible things being, of course, things that are perceived by the senses). With this established, Philonous is ready to begin. He will spend the first dialogue demonstrating that materialism leads directly to skepticism, and the second and third proving that his own idealism leads in the opposite direction, toward faith in common sense.
Berkeley is intent on setting himself up as the defender of common sense. As we move further into the work, and begin to gain an understanding of what his idealism entails, we will be able to assess Berkeley's right to give himself this title; for the time being, though, we can ask why he is so concerned to bestow it upon himself. Why does Berkeley care so much that his view be seen as the view of common sense? There are several levels on which we can answer this question.
On the most basic level, the clear answer is that Berkeley's view sounds so nonsensical at first read. Anyone who is claiming something seemingly radical, has a stake in proving that their view is actually the most sensible view in the world. And Berkeley's view definitely qualifies as radical, despite Philonous' protests to the contrary. What Berkeley is trying to get us to believe is that everything we see around us — tables, chairs, flowers, grass, sky, ocean, birds, cats, and so on — are all in our mind. They are ideas. They do not have any independent, absolute existence out in the world. Though, as we will see, his fleshed-out theory is actually more subtle and sophisticated than it might seem from this rough description, this is basically the gist of it: objects are nothing but collections of ideas.
Any sane person would balk at this theory, at least when first presented with it, and Berkeley knows this. He knows that this view sounds like skepticism at its finest: like the denial of an external world. If anything will keep people from buying into his theory, it will be this very feature: the fact that it seems so contrary to our common sense. So it makes sense for Berkeley to turn the tables on us, and try to demonstrate that, actually, this view that we judge as so ridiculously far fetched is actually the view that best approximates common sense. If he can get us to believe this, he will have crossed the biggest hurdle to getting his theory accepted.
But Berkeley also has another, deeper reason to set himself up as the defender of common sense: he really thinks that he is. Why, we might ask ourselves, would anyone even come up with a theory this crazy? Was he just trying to see what he could get people to believe? Was he engaging in a purely intellectual activity? Berkeley came up with this theory, specifically because he wanted to effect a return to the common sense principles he thought the philosophers had abandoned. He really believed his own rhetoric; he really believed that his idealism was the most common-sensical view in the world. Berkeley saw his theory as motivated by four common-sense principles. The first of these is the belief that we can trust our senses. The man on the street believes that what his eyes and ears and mouth and nose tell him about the world is trustworthy. He thinks that the world has colors, and sounds, and tastes, and smells, and feels just like those he experiences. When he sees a purple ball lying by a blue pool of water, he takes this as solid proof that there is, in fact, a purple ball lying beside a blue pool of water. The philosophers, or at least those who buy into the new mechanistic science, do not believe this. The philosophers think that the world is really made up of tiny particles of matter that have no color, sound, taste, feel, etc. (in short, none of the so- called secondary qualities). These tiny particles of matter move around in such a way that they produce in us the illusion of color, taste, and so on. The colorless particles in the ball, for instance, move around in such a way that our eyes perceive the ball as purple; the colorless particles in the water move around in such a way that our eyes perceive the water as blue. But the ball and the water do not really have any color at all.
The second common sense principle that Berkeley thinks he is defending is the belief that the qualities we perceive as existing really do exist. The man on the street believes that there is blue and sweetness and the sound of a trumpet in the world. The philosopher, as we have just seen, does not. The philosopher differentiates between secondary qualities (color, taste, smell, sound, heat), which do not really exist in the world, and primary qualities (size, shape, number, and motion) which do really exist in the world. Rephrasing the above philosophical picture using these concepts, we can say: it is the primary qualities of the tiny particles of matter that give rise to our (illusory) sensations of secondary qualities. Berkeley disagrees strenuously.
The third principle of common sense that Berkeley promotes, is the conviction that the things we see and feel are real. The man on the street does not doubt that the cars he is passing are real things. He does not doubt that the people he sees and hears passing by him are real. He does not doubt that the sun he sees overhead, and the cement he feels beneath his feet are real. The philosopher, in contrast, does doubt these things. The philosopher (at least Descartes and Locke) believes that the immediate objects of his perception are merely ideas, which are mental copies or representations of real things. The philosopher, therefore, does not think that we have any direct access to real things; what we perceive is only our own ideas, and through these we gain access to the real world of objects. This view of perception, on which ideas mediate between us and the world, is often called either the "mediated view of perception" or the "veil of perception view".
The veil of perception view can lead to another unfortunate conclusion: if all we see are our own ideas, we can begin to doubt that there even are any real things out in the world that resemble our ideas. We can begin to worry, as Descartes would have us do, that all of our ideas are caused by an evil demon. Or, to put a more modern spin on the worry, we can wonder whether we are just a brain in a vat, and all of our sensations of the world are caused by a mad scientist, who is electrically stimulating our nerve endings with a computer. In short, we can begin to doubt whether there are really any flowers, trees, sun, moon, and sky around us at all. Therefore, the final principle of common sense that Berkeley wants to defend is the belief that all skeptical doubt about the real existence of things is unjustified.
Berkeley thinks that the best way to defend these four principles — (1)that we can trust our senses, (2) that the things we see and feel are real, (3) that the qualities we perceive as existing really do exist, and (4) that all skeptical doubt about the real existence of things is, therefore, precluded — is to claim that there is no such thing as matter. It is for this reason, above all, that he proclaims himself the defender of common sense.