Substance, nature, and necessity › How the argument reaches one substance

Spinoza does not begin by declaring one substance an axiom. He argues toward it. Part I, proposition 5 denies that two substances can share an attribute. His proof considers whether substances could be distinguished by attributes or by their modifications. Since substance is prior to its modifications, differences between modifications cannot, he argues, distinguish substances considered in themselves.

This is a substantive argument, and its premises deserve attention. A critic can ask whether setting aside modifications removes every relevant basis for distinguishing substances, or whether the way attributes identify substance has already done too much work. Putting the argument into a numbered proof makes those commitments easier to locate. It does not establish them merely by giving them that form.

Definition VI describes God as absolutely infinite substance, consisting of infinite attributes, each expressing eternal and infinite essence. Proposition 11 argues that God necessarily exists. Proposition 14 then uses the earlier exclusion of shared attributes: if another substance existed, it would have to possess an attribute already belonging to absolutely infinite God. That would conflict with proposition 5. Spinoza concludes that no substance besides God can exist or be conceived.

Proposition 15 states that whatever exists is in God and cannot be conceived without God. Proposition 18 identifies God as an immanent cause, rather than a cause that produces something outside itself. Finite things therefore belong within the explanatory order of the one substance. They are not independent products standing outside the reality that makes them possible.

The phrase "God or Nature," used in the preface to Part IV, can help a reader recognize this departure from an external creator. It requires care, however. Nature here cannot simply mean a collection of familiar objects considered apart from the infinite substance through which they are understood. Nor is Spinoza describing a magnified human agent who chooses which world to make for our benefit.