Philonous has just presented both of these arguments as applied to the case of heat, and is about to move on and make the same arguments for taste, when Hylas breaks in with an objection. Until now he has been reluctantly agreeing with everything Philonous says, but now he realizes that he agreed to some things he did not really believe. Intense heat, he points out, is not the same thing as pain. Rather, it is the sensation of intense heat that is pain (once an object becomes hot enough, you cease to have a sensation of heat and have a sensation of pain instead). However, there is also another aspect of heat in addition to heat as it is perceived by us: there is hear as it exists out in the fire. The first of these (the heat as it is perceived by us) is a mind-dependent idea, but the second has a mind-independent existence out in the world and it causes our sensation of heat.
Philonous, however, has a ready reply to this objection. He reminds Hylas that for the time being they are focusing exclusively on immediate perceptions. As far as immediate perceptions go, there is only the heat as it is felt by us. We have no immediate access to any other aspect of heat, and so this other aspect is irrelevant. What is important is that in our sensations it is impossible to distinguish intense heat from pain. We simply feel intense heat as pain.
Hylas back down, and Philonous resumes his project, applying the argument from perceptual relativity to the other senses. What tastes sweet to us at one time, might taste bitter to us at another. (Think of orange juice before and after you brush your teeth). Colors change too, depending on the lighting conditions. Philonous goes on like this for a while, demonstrating that none of these qualities can possibly exist in material objects, but must exist in the mind.
Hylas is now completely won over on the point that colors, tastes, smells, and all other secondary qualities exist only in the mind. However, he begins to wonder whether primary qualities such as shape, size, and motion, might have some independent existence out in the world. Philonous, however, has perceptual relativity arguments for these qualities as well. What seems small to us seems large to a mite, so size cannot be inherent to material objects. What seems one shape from one angle appears to us as another shape from a different angle. Yet we do not think that the same material object can have different shapes, so the shapes too must belong to the mind and not any mind-independent object. Motion is also perceptually relative: the same object in motion can appear to be moving slowly or quickly, depending on how slowly or quickly I myself am moving. Motion too, then, cannot belong inherently to a mind-independent material object.
At this point, Philonous feels that he has successfully convinced Hylas that no sensible qualities exist out in the world, independent of mind. He has collapsed the traditional distinction between quality and idea, showing that qualities just are mind-dependent ideas.
Berkeley is not the first philosopher to suggest that the secondary qualities are mind-dependent. As we saw in our analysis of the first section, philosophers since Descartes have been distinguishing between primary qualities and secondary qualities, and claiming that while the first class belong to objects out in the world, the second class do not. Locke himself seems to waffle on the subject of secondary qualities. At times, he speaks of them as if they existed in objects as powers to produce certain sensations in us, and at times he speaks of them as if they really were just those sensations — as if blue, for instance, were nothing other than our sensation of blue. Berkeley, then, is not yet treading on radical ground when he argues that secondary qualities are mind-dependent. He is, however, breaking free from tradition when he claims that primary qualities, as well, belong to our mind. He is collapsing yet another Lockean distinction (though not a distinction unique to Locke): the distinction between primary and secondary qualities.
Because this distinction is popular, Berkeley feels that he must account for the fact that so many philosophers have been tempted by it: if there is no difference between primary qualities and secondary qualities, why have so many smart people thought that there is a really significant difference? Berkeley's answer, which he gives at 1.192, is that philosophers were persuaded to the truth about secondary qualities by the fact that these qualities are so obviously connected with the unquestionably subjective qualities of pleasure and pain. Since the primary qualities lack this connection with pleasure and pain, the philosophers failed to recognize that these, too, are mind-dependent.
Turning now to Hylas' attempted objection to the argument from pain, we must ask what Hylas meant to be pointing out and whether Philonous' reply was successful. What Hylas meant to draw Philonous' attention to is this: we use the word "heat" to mean two different things: what heat feels like to us, and some molecular activity in the object. Each of these meanings captures some of our intuitions about what heat is, and so we do not want to give up either. We think that these are two equally real aspects of heat — the mind-dependent aspect (heat as perceived by us), and the mind-independent aspect (heat as it exists in the object). But Berkeley, as we saw in the last section, is an empiricist. Therefore, he is able to say that, when we are limiting ourselves to talking about immediate perception, we are also limiting ourselves to talking of heat only in the first aspect. All that we have immediate access to is heat as it feels to us — that is, our sensation of heat. If there is such a thing as heat as it exists in the fire, then this is only mediately perceived and so not relevant to the topic at hand.
But we might press Berkeley on this point: Do we really only mediately perceive heat as it exists in the fire? Maybe our sensation of heat is just a perception of heat as it exists in the fire. In other words, perhaps what it is to perceive heat-as-molecular-motion is just to have a certain sensation. In that case, we are immediately perceiving the heat-as-molecular-motion when we have our sensation of heat. Berkeley knows that this is a move his critics can make, and he actually uses it to further his argument. He points to the case of sound: philosophers like to distinguish between sound as it is heard and sound as it exists as the motion of air particles. This is analogous to the distinction between heat as it is perceived by us, and heat as it exists as molecular motion in the fire. But, he asks, does it really make any sense to say that we "hear" the motion of air particles? The motion of air particles is the sort of thing that you could see or touch, but not the sort of thing you could hear. So how could we say that what sound really is is the motion of air particles, if that is not even the sort of thing you could hear? We would actually be asserting that real sounds cannot be heard! When we immediately perceive sound, we are not thereby immediately perceiving the motion of air particles, since these are not even the sort of things that can be heard. Similarly, he would say, when we immediately perceive heat we are not thereby immediately perceiving molecular motion because this is not the sort of thing that can be felt as hot.