I turn now to discuss negation. This is a subject that runs through a good many of Spinoza's positions, and I think there are certain preconceptions concerning it which are at work in much of Spinoza's reasoning. The notion of negation crops up fairly early in the Ethics – in fact, in the explanation of Definition 6. Definition 6 is a definition of God as a being absolute infinite, and the explanation runs,
I say absolutely infinite, not infinite of its kind. Of a thing infinite only of its kind, infinite attributes may be denied, but that which is absolutely infinite contains in its essence whatever expresses reality and involves no negation.
The central presumption here is that anything of which we have to say that it is not something in order to give a complete account of it, of what it is, is not substance. Because in saying that there is something which it is not, I think Spinoza would understand this as saying there is something else which it is not, and so we require a reference to something else, other than the nature of that which we are trying to explain. But if we require a reference to something else in order to give that explanation, then that which we are explaining the nature of can't be substance, because it's being conceived through something else and not through itself.
So the thought here seems to be that anything which requires a negation in order to be explained is something that requires that we understand the nature of something else in order to understand it. Hence it can't be substance. Essentially the same point can be put, and is put by Spinoza in various places, by saying that anything, the account of which involves negation, is limited by something else. And it seems to me that all of Spinoza's arguments (or most of them anyway) against the possibility of a plurality of substances turn on this point in one way or another. If there were two substances, A and B, then there would be something that was not true of A but true of B, otherwise they’d be indistinguishable and we couldn't say they were two.