Philonous' project begins with an ambitious first goal: he must show that we have no reason to believe in the existence of mind-independent material objects. He tackles this goal in two stages: first he will show that we are never presented with any mind-independent material objects in our immediate experience (that is, through our senses), and then he will show that we have no reason to make an inference from our immediate experience to the existence of mind- independent material objects. Because Berkeley is an empiricist, he believes that all our knowledge comes through sensory experience; he, therefore, feels secure in the fact that by demonstrating that we do not have evidence of mind- independent material objects either through immediate sensory experience, or through inferences based on this sensory experience, he is actually showing that we have no evidence of the existence of mind-independent material objects at all. There is simply no other way to acquire knowledge on his view.
In order to prove that we do not have any evidence for the existence of mind- independent material objects in our immediate sensory experience, Philonous presents two arguments: the argument from pleasure and pain, and the argument from perceptual relativity. Before launching into either one of these arguments, though, Philonous must lay some groundwork. First, he asks Hylas to admit that all we immediately perceive of an object are its sensible qualities. Hylas readily assents to this claim. It is true by definition that the only thing we sense are sensible qualities; anything else is not sensibl. Philonous next presses Hylas to admit that sensible things themselves are nothing but collections of sensible qualities. Hylas hesitates a little; he believes that there is something else to objects other than their sensible qualities, something like their hidden microstructures. But Philonous assures him that he only means to say that sensible objects are collections of sensible qualities insofar as they are sensible. Insofar as a cherry is sensible, for instance, it is just an amalgamation of redness, smallness, sweetness, etc. It may well be something other than this amalgamation, though, in actuality. Convinced, Hylas consents to this claim as well.
Philonous has managed to get Hylas to agree that the only things we immediately perceive are sensory qualities. Now he needs only prove that these qualities are mind-dependent and he had shown that that everything that we immediately perceive is mind-dependent. In other words, he can now demonstrate that we have no evidence for mind-independent material objects in our sensory experience. He will then have mastered the first stage in his project. This is where the argument from pleasure and pain and the argument from perceptual relativity come into play.
Philonous starts with the idea of pain. In the case of pain it makes perfectly good sense to say that it cannot exist outside of the mind, or, as Philonous puts it, its existence is to be perceived (in Latin "esse is percipi"). After all, how could pain exist if no one is feeling the pain? What pain is fundamentally involves its being felt. The same is true of pleasure. In order to show that this is true not only for pleasure and pain, but for all the other sensible qualities too, Philonous tries to demonstrate that there is an extremely tight connection between the other qualities and these two qualities: that, in fact, it is impossible to separate the other secondary qualities from pleasure and pain. Since pleasure and pain cannot possibly exist outside of the mind, and the other qualities are inextricably linked with pleasure and pain, goes the argument, none of the qualities can exist outside the mind.
The first sensible quality that Philonous tries to link with pain is intense heat. Intense heat, he tells us, is simply felt as pain. What it means to feel intense heat, is to feel pain. So since pain can only exist in a sentient being, the same is true of intense heat. Intense heat, then, is mind-dependent. In detail the argument goes like this:(1) Nonsentient things do not experience pain and pleasure.(2) Matter is nonsentient. (3) Matter is not capable of pleasure and pain.(5) Intense heat is a form of pain.(6) Hence matter is not capable of feeling intense heat.(7) So intense heat is mind-dependent. (7) Finally, since intense heat and all other degrees of heat must be the same type of thing, all degrees of heat must be mind-dependent. After all, it would be unlikely that, as heat moved up in degrees, it suddenly went from outside the mind to inside.
The argument from perceptual relativity argues for the same conclusion: that sensible qualities can only exist inside the mind, and cannot belong to matter. (1) The same thing cannot be both cold and warm at once. (2) Material things that are perceived to have a moderate degree of cold or warmth are really cold or warm. This is a materialist assumption. (3) The same water can be perceived to be cold to one hand and warm to the other. For instance, imagine that one hand was just in the freezer and the other in the oven. Now you stick them both in the same bucket of lukewarm water. To the hand that was in the freezer the water feels warm, and to the hand that was in the oven the water feels cold.(4) So the same water is both cold and warm at once.(5) Therefore, the cold or warmth cannot belong to a material object (i.e. mind-independent water), since the same thing cannot be both cold and warm at once. Instead, we must say that the heat, warmth, cold and so on, really belong to the perceiver, that is, to the mind, and not to the water.
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.
But here is another objection that can be made: it is not true that there is no way to tell which of these impressions is veridical; we can use a thermometer to take the temperature of the water, and thus verify whether it is hot or cold. Berkeley has a reply to this as well, and once again this reply depends on his empiricism. If we need to use fancy instruments, he would tell us, then we are not talking about what is immediately perceived. According to the empiricist, what is immediately perceived is only what comes to us directly through our senses. And it is only the heat we feel that comes to us through our senses, not any information that we receive from a thermometer.