For Spinoza there is only one substance and this substance is God. God, too, is central to Descartes’ philosophy, for without the proof of the existence of God his whole metaphysics would collapse. But to some extent he still has a theological conception of God. God is understood as separate and transcendent in relation to the world such that Descartes splits substance between the infinite and finite, and finite substance itself is split between extension and thought. Spinoza is precisely rejecting this split when he writes in the Ethics that: ‘God is the immanent, not the transitive, cause of all things’ (1P18). We can understand Spinoza’s metaphysics as deducing the necessity of there being only one substance from Descartes’ principles. Therefore it is not simply a matter of Spinoza rejecting Descartes’ philosophy, but of demonstrating that following his own principles he too must agree that there can only be one substance and that this substance must be God.
The relation of Spinoza and Descartes to the idea of God is itself ambiguous. On the one hand they both agree with the essential definition of this idea that God is supremely perfect and infinite being. This means that God cannot be conceived as limited in any way since he would be less than perfect if he were so. Already in this idea, therefore, lies the necessity of one substance. For if, like Descartes, we do make a split between infinite and finite substance, then we are limiting God, namely by contrasting and opposing God to the created world, which has its own independent existence (and must do if we are to call it a substance). The only way Descartes can get out of this contradiction is by producing another one by arguing that finite substance must be dependent on God’s power for its own existence, which would mean that finite substance would be both dependent and independent at the same time. For Spinoza the very idea of a dependent substance, following from Aristotle, is a contradiction in terms.
We can still see, however, that even with this abstract definition, which is the same for both Descartes and Spinoza, Descartes’ philosophy is still caught within a theological definition of God (a human fiction for Spinoza, following the appendix of the first part of the Ethics). This is because Descartes is still willing to talk about God in terms of divine attributes, such as omnipotence and omniscience that distinguish God from the created world. There is a separation between what is created and the creator. God is special kind of substance in relation to the substance of the world. Thus the idea of creation is still central to Descartes’ metaphysics, which would be completely meaningless for Spinoza. In fact we might think of Spinoza’s metaphysics as the final expulsion of any idea of creation from philosophy (in the appendix to Part 1, Spinoza writes about this idea through the fiction of final causes, where nature is imagined to be created for the benefit of humankind by a tyrannical God, as opposed to being considered in terms of its essence).
Descartes still exists in the theological conception of an absolute separation, division or opposition between the world, on the one hand, and God, on the other. For Descartes, therefore, God cannot be extended, because God and the world are entirely different substances. How would Spinoza counter this theological conception? Again, following the appendix to the first part of the Ethics, he would say that we must start with the essence of things, rather than what people might imagine things to be, and that this is the same with the idea of God, as of anything else. What many people say of God – He is good, omniscient, omnipotent, and so on – are properties, but they do not say what God is in terms of attributes; that is to say, in terms of his essence. Take, for example, of omniscience. This is a property of God, but it cannot be a ‘fundamental property’, since it presupposes the attribute of thought on which it is dependent (it is impossible to conceive of an ‘all-knowing being which does not think).
Descartes would probably not disagree with this argument, but it is clear that he would not accept that God could be conceived of in terms of extension, since extension is not infinitely perfect for Descartes. This is because extended matter is divisible, and it is clear that God cannot be. Why does the divisibility of matter imply imperfection for Descartes? This is because divisibility is the destruction of matter, and destruction is an imperfection. Spinoza’s argument against this is that divisibility of matter is merely a mode, and in essence, matter is not divisible. This is because following 1P5 there can only be one extended substance, since two or more substances cannot have the same attribute, since they would not be anything that would distinguish them: ‘In Nature there cannot be two or more substances of the same nature or attribute.’ If there is only substance, can we really say that matter is being destroyed? Even if I divide extended substances into different parts, nonetheless these different parts still exist as part of the one substance, which has not been destroyed at all. The leg that I cut off the horse is no longer part of the horse, but both the horse and the leg are still part of one and the same substance, and therefore I have not separated this substance, when I have separated the horse from its leg. If this were the case, then the separated leg would no longer belong to extension at all. In fact the horse’s leg is just a portion of extension that is qualified in a certain way. We should not, therefore, confuse the disabling of matter with its destruction. It is also means that extension is as infinite and eternal as thought, and it is only a theological prejudice of Descartes that prevents him from saying that it is just as much a ‘fundamental property’ or attribute of God as thought.
Spinoza’s philosophy is not a refusal of Descartes’, but is a thorough logically worked out consequence of his thought, which Descartes could not himself go to the end of perhaps because of a theological prejudice which prevented him from understanding these consequences. For Descartes, each substance has one attribute which constitutes its essence. For minds it is thought, and for bodies, extension. He calls these ‘principle attributes’. Knowing what the principle attributes are would tell you what you are dealing with, and what you should expect. Thus, you would not be lead to the mistake of confusing and thought with a thing. One could say for Descartes, therefore, that what is important is not substance, but attributes, since attributes are the ‘principles of explanation’. For Spinoza it is the other way around. It is substance itself which is the principle of explanation, and not attributes, since it is not limited to the two attributes which Descartes describes, but must contain infinite attributes. Since to argue otherwise would be to limit substance and thus contradict its infinite essence, as Spinoza writes in IP8: ‘Every substance is necessarily infinite.’
For Descartes each separate attribute, which must be conceived in itself, since we do not need to know what thinking is to know what extension is, and vice versa, implies a separate substance, since he understands substance through attributes. Why, then, does Spinoza argue that we should think reality the other way around, and say that there is one substance with infinite attributes? In response to this we might ask whether it is possible to think of one substance with infinite attributes, perhaps because we tend to think in the same way as Descartes. Spinoza’s answer to this question is that we already do so through the idea of God. Since God is by definition infinite, He must contain infinite attributes, since if he did not, then He would lack something, which would contradict his infinite essence. Moreover if God did lack something then there must be something that caused God to do so, which again would contradict his essence and the very meaning of substance as independent.
We might not be convinced by Spinoza’s argument at this point, but Curley says that there is another way to get to the same conclusion. If each attribute is conceived through itself, it must, therefore, also exist in itself. If it existed in something else in order to exist, then we would need to be able to conceive such a thing before we could conceive the attribute. If an attribute is conceived in itself, and exists in itself, then it satisfies the definition of substance in 1D3 (‘by substance I understand what is in itself and is conceived through itself’). But if we have infinite attributes, each conceived and existing in itself, wouldn’t we then have an infinite amount of substances, rather than just one, as Spinoza believes? Curley’s answer to this question is to say that Spinoza’s substance is a ‘complex of very special elements’ (Curley 1988, p.30). If each attribute is conceived through itself, they must also exist in themselves, and must also exist necessarily. If this is the case, then no single attribute could exist without the others, since they all necessarily exist: ‘The existence of each one of the attributes implies the existence of all the others’ (Curley 1988, p. 30). Substance, therefore, is not anything different from attributes. It isn’t something that lies behind attributes, as some kind of separate and distinct cause, which would lead us straight back to the transcendence we are trying to get rid of. God, therefore, is nothing but the existence of an infinite plurality of attributes, and nothing else.
Curley, E., 1988. Behind the Geometrical Method : a Reading of Spinoza’s Ethics, Princeton N.J.: Princeton University Press.
 Ethics, p. 3. See also IP13. To make matter divisible is to divide it into parts, but that would either mean that these parts would not be the same as substance, which would cause substance not to exist, or there would be many substances with the same attribute which would be absurd.