A Short History of Modern Philosophy: From Descartes to Wittgenstein, Second Edition by Unknown Author

to be what it is. He also noted an ambiguity in the term ‘substance’, which might be used in a wide sense, to denote any individual object, or in a restricted sense, to refer to that which depends upon nothing outside itself for its existence. In this restricted sense, he argued, only God is a substance.
    It is this restricted idea of substance that provides the cornerstone of Spinoza’s metaphysics. A substance, he writes, is ‘in itself and conceived through itself’, or is ‘that the conception of which does not depend upon the conception of another thing from which it must be formed’. A substance must be intelligible apart from all relations with other things. Hence a substance cannot enter into relations and, in particular, can be neither the cause nor the effect of anything outside itself. To the extent that a thing is caused, it must be explained in terms of, and therefore ‘conceived through’, other things. A substance therefore cannot be produced by anything else: it is its own cause (causa sui )—which means, according to Spinoza’s definition, that its essence involves existence.
    Spinoza, evidently influenced by Descartes, distinguishes the attributes of a substance from its modes. An attribute is that which ‘the intellect perceives as constituting the essence of a substance’, whereas a mode is that which is ‘in something else’ through which it must be conceived. The word ‘in’ here creates difficulties, but here is an analogy: a group of people join to form a club which then does things, owns things, organises things. When I say that the club bought a house, I really mean that the members of the club did various things, with a specific legal result. But none of the members bought a house. Hence it looks as though the club is an independent entity, existing over and above the people who compose it. In fact, however, it is entirely dependent for its existence and nature on the activities of its members. The club is ‘in’ the members, in Spinoza’s sense. And when x is ‘in’ y, x can be understood fully only through y. Another way to put the point is: y is ‘prior to’ x, since we cannot understand x without a prior conception of y. In this sense, ‘a substance is prior in nature to its modes’.
    The first part of the Ethics is devoted to God, defined as ‘a substance consisting of infinite attributes, each one of which expresses an eternal and infinite essence’. Spinoza follows Descartes in giving a version of the ontological argument. However, the proof has an interesting twist to it. Spinoza believes that all substances exist necessarily, since ‘it belongs to the nature of substance to exist’. But he also argues that ‘there cannot be two or more substances with the same nature or attribute’; in other words, substances cannot share attributes. Since God possesses all attributes, therefore, there can be no other substance besides God. Everything that exists is ‘in’ God.
    God has ‘infinite attributes’. Extension is an attribute, since we perceive it as constituting the essence of the corporeal world: there is nothing more basic than extension to which the explanation of corporeal things could be referred. We have full (or, as Spinoza puts it, ‘adequate’) knowledge of the nature of extension through the science of geometry, and the existence of this systematic science of necessary truths is further proof that the idea of extension delivers God’s essential nature to our intellect.
    Monism
    Extension is an attribute of God, and like all the attributes of God it is infinite in quantity (which means, to put it crudely, that space has no boundaries, a proposition for which Spinoza provides an independent proof). It remains to examine what other attributes God might have. The other candidate bequeathed by Cartesian philosophy was thought, which Descartes put forward as the essential characteristic of mind. Spinoza argued that this too must be an attribute of the