When people talk about the empiricists, they are usually contrasting them with rationalists. In particular, when the context is early modern philosophy, "the empiricists" refers to John Locke, David Hume, Thomas Reid, and Berkeley himself, while "the rationalists" refers to Rene Descartes, Nicolas Malebranche, Baruch Spinoza, and G. W. Leibniz. Aside from these neat divisions into camps, though, there is nothing neat about the distinction between empiricist and rationalist. Nearly every criterion that has been posited as the deciding issue between these groups, breaks down in one way or another when we inspect the philosophies of each side. For our present purposes, though, it is sufficient to understand the difference as this: for the empiricists, all substantive knowledge must come through the senses, whereas for the rationalists there is some knowledge that can be obtained by rationally inspecting our innate ideas using our purely intellectual faculty of thought.
Berkeley is confident as an empiricist that he has exhausted all the ways in which we could come to know about mind-independent material objects. Because Berkeley is an empiricist, he believes that all knowledge comes through the senses. Therefore, he thinks that there are only two ways that we can come to know about things: either we can have immediate evidence of something through our sensory experience (e.g. I know that you are on the stairs because I see you on the stairs), or else, we can make an inference based on our immediate sensory experience (e.g. I know that you are home, because I hear your voice from the stairway). If Berkeley were a rationalist, though, he would think that there is another option available: we could come to know about mind-independent material objects by inspecting some of our innate ideas, and seeing whether we can use these, along with our faculty of reason, to arrive at a proof for the existence of mind-independent material objects. This is, in fact, exactly the way in which Descartes argues for the existence of mind-independent material objects.
Berkeley does not even consider this rationalist option for proving the existence of mind-independent material objects. He is not at all interested in proving that this is not a viable option; he is simply content to ignore it. His primary target in the Dialogues is his fellow empiricist, John Locke. In fact, Berkeley's tendency to ignore the challenges which rationalism poses to his system, is probably due in large part to the great pains which Locke himself took in the Essay Concerning Human Understanding to debunk the rationalist model of knowledge acquisition. In that work, Locke attempted to prove that there is no such thing as innate ideas, and to establish that all substantive knowledge comes through the senses. Berkeley is willing to rest assured that Locke finished off that job sufficiently; he thinks that Locke freed him from having to even worry about rationalism as a plausible threat.
It is not uncommon to find a philosopher fixating on one main opponent in his work, but the extent to which Berkeley fixates on Locke is unusual. The Australian philosopher David Stove goes so far as to suggest that Berkeley was so fixated on Locke as his opponent, that he made the fatal mistake of believing that any argument against an element of Locke's philosophy, was an argument for his own idealism. In other words, he believed that there were only two options: his own system and Locke's system, and so proving Locke's system wrong amounted to proving his own system right. This claim seems a little drastic (and uncharitable), but it does help to underscore just how integral Locke was to Berkeley's thought.
Locke's philosophy is a guiding force for Berkeley. A good deal of the work in Three Dialogues is devoted to conflating three Lockean distinctions: the distinction between ideas in the mind and qualities out in the world, the distinction between observable qualities and the unobservable substratum, and the distinction between the nominal essence of objects and their real essence.
In this section, Berkeley tries to conflate the first of these distinctions. The common sense understanding of what sensible qualities are, he argues, is more like what Locke calls "ideas" than what Locke calls "qualities". As we have seen, Berkeley argues for the collapse of this distinction through two different arguments, the argument from pleasure and pain, and the argument from perceptual relativity. There are various objections that can be advanced against both of these arguments (we will look at some of these objections in the coming sections) but for now, we will focus on only two objections, both of which apply to the argument from perceptual relativity. The first objection is that all that we can conclude from the fact that one hand feels the water as cold and the other as hot, is that the water is not both hot and cold; there is no reason, however, to say that the water is neither hot nor cold. Berkeley is aware of this objection, and actually uses it to further his argument. If there is no good reason to decide that one or the other of these judgements is right, he explains, by the principle of parity, or fairness, we must decide that neither one is right. The water is neither hot nor cold.