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.