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.
Berkeley actually begins the Principles with an extended attack on abstract general ideas; in the Dialogues this argument is shortened considerably and given much less prominent billing. These changes can probably be attributed to a very simple motive: in the Dialogues Berkeley is trying to make his philosophy more accessible and popularized, and the concept of abstract general ideas is rather abstract and obscure. In short: abstract general ideas were neither sexy enough nor fun enough to make the cut. Nonetheless, they play an important role in many of Berkeley's coming arguments.
So first, what are abstract general ideas, and why did Locke posit them? The reasoning behind abstract general ideas was this: everything that exists in the world is particular. There is no such thing as just plain dog in the world; there is only Rex, Rover, Spike, Fido, and so on. So, Locke asked, how do we get our ideas like "dog", "cat" and "flower"? His answer is that we arrive at these general ideas by abstracting away from the particular ideas. For instance, to return to our example of dogs: from my contact with Rex, Rover, Fido, and Spike I receive the ideas "Rex", "Rover", "Fido", and "Spike". Now I can take these ideas of particular dogs and focus on what is similar in all of them: the tail, the shape, the bark, the fur, etc. I then abstract away these similar features from all of the particularizing differences and arrive at an abstract general idea of dog. I can do the same thing for "cat", "man", "hat", and anything else.
Berkeley thinks that this procedure makes no sense, as we will see, but he also thinks that this procedure cannot be Locke's whole story. Though Locke talks as if this act of isolating the similarities from the differences is the only process involved in forming abstract general ideas, this cannot be true. There must also be an even more incoherent process involved: a process of generalization, in which you take away all details and leave only the most vague idea. Take, for instance, the abstract general idea of color. According to Locke's theory, we must derive this idea from our ideas of redness, blueness, pinkness, greenness and so on. But we cannot treat these ideas as we did the ideas of Rex, Fido, and Rover. There is nothing in common that we can attend to; all there is to these ideas is their color, and we must ignore each particular color if we are to arrive at the idea of color in general. There is no determinate similarity between these ideas. So we need a generalizing process. But, Berkeley points out, how would this work? If we sacrifice all of the determinate details, then we have nothing to build our idea with. The process of isolation is really no better: we will end up having to add in so many particular details of the dog that our idea will become helplessly confused and useless. In short, there are only two ways to form an abstract general idea: either take all the detail out, or put all the details in. Neither is feasible. If you take all the detail out there is nothing determinate with which the build the idea; you will have an idea with shape but no particular shape, color but no particular color and so on. If, on the other hand, you put all the detail in, you arrive at an incoherent chaos, with a million shapes, colors, types of fur, or whatever.
Locke was working with two assumptions. The first of these is the content assumption. According to the content assumption it is the immediately present content of an idea that determines what in the world the idea signifies (i.e. the object of reference of that thought). Whatever is before your mind, determines what you are thinking about. To be thinking about Fido, for instance, you must have the idea "Fido" before your mind. To be thinking about dog in general, you must have the idea "dog" before your mind. This, in itself, would not have gotten Locke into trouble, but his second assumption was that ideas must be images. So my idea of Fido must be a picture of Fido, and my idea of dog must be a picture of a dog.
Now we can see why Berkeley thinks that abstraction is impossible: you cannot possibly have a picture of a dog that either has all the similarities between dogs thrown in, or all the details taken out. Such a picture could not exist. If Locke had gotten rid of either of these assumptions, he could have salvaged the notion of abstract general ideas. Today most people reject the second of these assumptions; they say that ideas do not need to be imagistic. Instead, they posit that some of our ideas are purely intellectual, discursive thoughts. This move, though, would not have worked for an empiricist: if ideas come from experience they must be imagistic.
Berkeley's tactic is to works with the first assumption instead. The difference between the idea of Rex and the idea of dog, he tell us, has nothing to do with what is immediately present to me. What is immediately present to me is the same in both cases; the difference comes in how I use the idea. I can either use the idea to refer to one particular dog, or to many particular dogs. In either case, though, it is the same picture before my mind.
Turning now to the notion of substratum, we must ask why Locke posited this idea, and how Berkeley defeats it. It is important to point out, first of all, that Locke himself was never totally comfortable with the idea of substratum; in several instances Locke uses language that would suggest he does not really believe substratums exist out in the world, that our idea of substratum refers to nothing and thus is meaningless. But Locke felt that he needed to include it in his philosophical system nonetheless. He gives us the following picture of the origin of our ideas of substances: As we go through the world we carve up the dense sensory array into discrete objects, noticing which qualities seem to regularly cluster together. For instance, we see softness, blackness, a certain small size, a certain catlike shape moving all together throughout our experience and we assume that all of these qualities make up a single object. However, he claims, this cluster of our ideas of observable qualities cannot in itself form the idea of a substance. We must also add to this an idea of whatever it is that these properties belong to; we do not simply believe that these properties exist out in the world, but rather that they are properties of something. That something, he argues, corresponds to our idea of substance in general, or substratum.
It is helpful to think of a substratum as an invisible pincushion, with all of the observable qualities that belong to it as the pins. The substratum itself is unobservable (and, hence, because of Locke's empiricism, unknowable) because it cannot itself have any observable qualities; it is the thing in which observable qualities inhere. Anything we can observe or describe is a property rather than the substratum itself. Our idea of substratum, therefore, is necessarily very obscure and confused. All we really know about substratum is that it is supposed to support the observable properties of a substance. Beyond that we have no hint and no hope of getting a hint.
Given how bleak Locke's view of substratum was, it is not surprising to find Berkeley attacking it. Berkeley's first line of attack, in fact, is taken straight from Locke himself: since a substratum is in principle unobservable, why on earth should we believe it exists? We can never get an idea of it through the senses, and we could never even form an idea of it through our reason. How, after all, could we form a positive idea of something with no qualities? His second line of attack — i.e. that a substratum must have extension to be a support, and extension is mind-dependent — is uniquely his own, and considerably weaker as an objection. As Hylas points out, Philonous is unfairly interpreting the term "spread" in a literal sense, and the fact that Hylas himself cannot come up with a better interpretation of the word does not mean that there is not a better interpretation out there. More importantly, though, this argument crucially depends on our buying Philonous' previous claim that extension is mind-dependent, which few of us, presumably, really do. In this case, though, Berkeley is able to rest on the weakness of his opponent: though Berkeley's own arguments against substratum are weak, the arguments he borrows from Locke are strong. Locke is defeated, in this instance, by his own words.