Benedict de Spinoza, Ethics, Proposition XIV and XV
Besides God no substance can be granted or conceived.
Proof.—As God is a being absolutely infinite, of whom no attribute that expresses the essence of substance can be denied (by Def. vi), and he necessarily exists (by Prop. xi); if any substance besides God were granted, it would have to be explained by some attribute of God, and thus two substances with the same attribute would exist, which (by Prop. v) is absurd; therefore, besides God no substance can be granted, or, consequently, be conceived. If it could be conceived, it would necessarily have to be conceived as existent; but this (by the first part of this proof) is absurd. Therefore, besides God no substance can be granted or conceived. Q.E.D.
Corollary I.—Clearly, therefore: 1. God is one, that is (by Def. vi) only one substance can be granted in the universe, and that substance is absolutely infinite […].
Corollary II.—It follows: 2. That extension and thought are either attributes of God or (by Axiom i) accidents (affectiones) of the attributes of God.
Proposition XV. Whatsoever is, is in God, and without God nothing can be, or be conceived.
Proof.—Besides God, no substance is granted or can be conceived (by Prop. xiv), that is (by Def. iii) nothing which is in itself and is conceived through itself. But modes (by Def. v) can neither be, nor be conceived without substance; wherefore they can only be in the divine nature, and can only through it be conceived. But substances and modes form the sum total of existence (by Axiom i), therefore, without God nothing can be, or be conceived. Q.E.D. […] All things, I repeat, are in God, and all things which come to pass, come to pass solely through the laws of the infinite nature of God, and follow (as I will shortly show) from the necessity of his essence.