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.