Descartes vs. Spinoza on Substance and Attributes
Descartes divided the world into two kinds of substances -- (1) res cogitans, or thinking substance, or mind, or soul, and (2) res extensa, or extended substance, or body. He was a substance dualist.
Descartes further divided thinking substance into two kinds:
(1a) infinite thinking substance (God)
(1b) finite thinking substance (each mind or soul)
There is no such division of extended substances; all extended substances are finite extended substances.
Each finite thinking substance (i.e. each a particular mind or soul) and each finite extended substance (i.e. each particular body) is a created substance.
Although Descartes believed that there was only one infinite substance (God), he believed that there were lots and lots of finite thinking substances, and lots and lots of extended substances. So he was a pluralist about the number of finite substances that exist.
(a) In some of the things that he wrote, Descartes did suggest that perhaps there was only one extended substance, that is, only one body (all extended matter). But he never said that there was only one thinking substance, that is, only one mind.
(b) In some things that he wrote, Descartes did concede that, properly speaking, only God or infinite thinking substance is a substance, since substance (after Aristotle) is that which is independent and does not require anything else for its existence, and only God is truly independent, since everything else is created by Him.
Descartes's God creates other finite substances, both mind or souls (finite thinking substances) and bodies (finite extended substances). So God does, in that sense, enter into transitive, or external, causal relations with other (finite) substances. This causal relation is purely one way, of course.
Spinoza rejects both of Descartes's divisions of substances, and rejects Descartes's plurality of finite substances.
First of all, he rejects Descartes's distinction between (1a) infinite and (1b) finite thinking substance. For Spinoza, there is only infinite substance, and no 'finite substance' (that is a contradiction in terms).
Secondly, he rejects Descartes's distinction between (1) thinking substance and (2) extended substance. For Spinoza, there are not two types of substance. There is only one type of substance. This type is infinite substance. To put it crudely, there is only one type of substance: substance. Substance is infinite.
Thirdly, there is only one instance of substance. That is, there is only one infinite substance. Only one substance exists.
In Spinoza's metaphysics there is only one substance. But, importantly, this one type of substance is not thinking substance, and is not extended substance. (It is not mind, and it is not body).
Substance has an essence. Any essence can normally be conceived of in a certain way. Substance, however, has an infinite essence. Hence it can be conceived of in an infinite number of radically distinct ways. (Or, if you don't like that, it has an infinite number of 'essences', and 'each essence' can be conceived of in a certain way). A way of conceiving of the essence of a substance is an attribute. Attributes are exclusive: there can be no overlap between two or more radically distinct ways of conceiving the essence of a substance. One attribute of substance is thinking. Another is extension.
Substance, then, can be conceived of as thinking, or conceived of as extended. It is not the case that it is to be identified with thinking, or identified with extension. (Otherwise, it would be stuck at being thinking only, or extended only). Rather, looked at one way, it's thinking, and looked at another way, it's extended. These are, as it were, possible perspectives, or 'interpretations', of substance. There are an infinity of possible perspectives, or 'interpretations', of substance. But we only happen to know about two such perspectives or 'interpretations': as thinking and as extended.
(Consider this non-Spinozistic example to explain attributes: The White House. One can conceive of this as a building. Or one can conceive of this as, say, the centre of political power in the US. These are two radically distinct ways of conceiving of The White House. Both are 'correct'. It is a building, and it is the centre of political power. But these are two radically different ways to conceive of the same thing, The White House. Neither makes the other wrong).
(But a better analogy would be: consider "Schnee ist weiss" and "La neige est blanche". Both of these sentences are different ways to express the same thing. In English, it would be said as "Snow is white". But the actual proposition is not in any one language. It is a proposition that is expressed in all possible languages.)