On the very first page of Spinoza's Ethics we find the perplexing definition of ‘attribute’: ‘By an attribute I mean what the understanding perceives in regard to a substance as constituting its essence’. Each attribute of a substance by itself thus constitutes the essence of a substance; if there are many attributes of the same substance, it does not take all of them together to constitute its essence. Spinoza, as we all know, in fact held that there is only one substance, God, but there are infinitely many attributes, of which only two, Thought and Extension, are accessible to the human mind. Each attribute, we further learn, has to be conceived on its own account (I, prop. 10); being conceived on its own account is, however, a distinguishing mark of the one substance, so how is it that the many attributes, which Spinoza says are really distinct, are not so many distinct substances, so many gods?
That is the ontological side of the puzzle. Now for the logical or grammatical side — about which writers on Spinoza have, I think, said a great deal less, though it has been much discussed as regards less deviant theology than Spinoza's. Each attribute is clearly meant to be a concrete, active, individual entity; yet the attributes are designated by abstract nouns — ‘Thought’ and ‘Extension’. Now can we make sense of such a sentence as ‘God is Thought’ or ‘God is Extension’, as opposed to ‘God thinks’ or ‘God is extended’? What does it mean to predicate an abstract noun of a concrete individual? And if this ‘is’ here is not a bare copula of predication but an identity sign, then how can we avoid passing from ‘God is Thought’ and ‘God is Extension’ to ‘Thought is Extension’? Spinoza would deny the conclusion, and it is quite essential to his system to do so. For if Thought just is Extension, identically so, then any mode of the attribute Thought is a mode of the attribute Extension and vice versa. But for Spinoza, the last is diametrically opposite to the truth: no mode is a mode of more than one attribute, and indeed no causal relations link modes of different attributes — a causal linkage is always confined to one attribute.