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Hylas, though, is not ready to let the discussion come to an end. He has thought of another objection. "Big", "small", "fast", and "slow", he points out, are relative terms. It makes sense, therefore, that no object is inherently big or small, fast or slow. However, philosophers also talk about something called "absolute size", "absolute extension", and "absolute motion". These ideas are abstracted from the relative ideas of "fast", "slow", "big", "small" and so on. Why can a mind-independent object not have absolute extension, size, or motion?
Philonous is unimpressed with this objection. What could it mean, he asks, to talk of "extension in general", "size in general", or "motion in general" abstracted out from any particular big, small, fast, slow, square, circular things? All that exists out in the world are particulars, and so these general qualities cannot exist in any corporeal substance. In addition, he argues, the very notion of these sorts of abstract general ideas is nonsensical. Can Hylas himself form an idea that is devoid of all sensible qualities such as swiftness, slowness, roundness, squareness and all those others that exist only in mind? Hylas admits that he cannot form any such idea. In fact, when pressed, he admits that he cannot even form an idea of extension, size, or motion devoid of secondary qualities such as color. Since he cannot form any such idea, Philonous concludes, he cannot have any such idea. There can be no idea of absolute extension, size, or motion and so it is nonsensical to even bring up these notions.
Hylas concedes the point, but he thinks that he has thought of yet another way to escape Philonous' bizarre conclusions: we need to distinguish the act of perception, which has no mind-independent existence, from the object of perception, which does have a real existence outside of the mind. Philonous, though, does not believe that this distinction can be made. First of all what act can we be talking about here? Our minds are not actively engaged in sensation, they are passive. We cannot choose what sensations we have. Plus, if you think you have to distinguish the active perception and the passive perceived in every sensation, what can you say about pain? How could pain exist outside of the mind, separate from the act of perceiving it? So clearly there is no such distinction to be made.
Hylas, undeterred, tries to formulate another objection. But when I look at the world and experience all these qualities, he pleads, I cannot help supposing that they are qualities of something — that they exist in something out there in the world. That is, he cannot help but believe in some material substratum, that acts as a support for all the sensible qualities. This attempt of Hylas' actually marks a major turning point in the dialogue: by arguing for the existence of matter as substratum, he is no longer trying to prove that we have immediate evidence for the existence of mind-independent material objects in our sensory experience. All that we have immediate experience of, after all, are observable qualities, and a substratum cannot be observed, since it is merely the support for all the observable qualities. Hylas has brought us into the second part of the analysis: whether we can infer the existence of mind-independent material objects from our immediate experience, rather than have their existence merely shown to us immediately through this experience.
Philonous has a great time demolishing the idea of substratum. Where would we get this idea of substratum, he asks? Obviously not through our senses, for the reason just mentioned: as the support for all sensible qualities, the substratum itself is in principle insensible. So it must be through reason. But we do not have a positive idea of substratum through reason: we cannot picture it, or describe in any precise way what it is. If anything, what we have is a relative idea of substratum as "whatever it is that supports qualities". Well, what would this mean? If it is the support of sensible qualities, then it must somehow be spread under the sensible qualities. But to be spread, something must have extension, and extension itself is a sensible quality, meaning that it cannot belong to substratum. The whole idea, then, is incoherent. Hylas points out that it is not really fair to understand the term "spread" in a strict literal sense, as requiring extension, but Philonous presses him to posit a more plausible sense for "spread", and Hylas cannot. He gives up the case for matter as substratum.
In this section, Berkeley attacks two very Lockean concepts: the concept of abstract general ideas, and the concept of substratum. It is worthwhile for us to run through each of these discussions in greater detail, getting a handle on the concept as it was presented by Locke, and on the argument against the concept as it is presented by Berkeley, both here and in the Principles.
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