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.