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.