%(a) Substance, Attributes, and Modes%.—There is but one substance, and this is infinite (I. prop. 10, schol; prop. 14, cor. 1). Why, then, only one and why infinite? With Spinoza as with Descartes independence is the essence of substantiality. This is expressed in the third definition: “By substance I understand that which is in itself and is conceived by means of itself, i.e., that the conception of which can be formed without the aid of the conception of any other thing.” Per substantiam intelligo id, quod in se est et per se concipitur; hoc est id, cujus conceptus non indiget conceptu alterius rei, a quo formari debeat. An absolutely self-dependent being can neither be limited (since, in respect to its limits, it would be dependent on the limiting being), nor occur more than once in the world. Infinity follows from its self-dependence, and its uniqueness from its infinity.
Substance is the being which is dependent on nothing and on which everything depends; which, itself uncaused, effects all else; which presupposes nothing, but itself constitutes the presupposition of all that is: it is pure being, primal being, the cause of itself and of all. Thus in Spinoza the being which is without presuppositions is brought into the most intimate relation with the fullness of multiform existence, not coldly and abstractly exalted above it, as by the ancient Eleatics. Substance is the being in (not above) things, that in them which constitutes their reality, which supports and produces them. As the cause of all things Spinoza calls it God, although he is conscious that he understands by the term something quite different from the Christians. God does not mean for him a transcendent, personal spirit, but only the ens absolute infinitum (def. sexta), the essential heart of things: Deus sive substantia.
How do things proceed from God? Neither by creation nor by emanation. He does not put them forth from himself, they do not tear themselves free from him, but they follow out of the necessary nature of God, as it follows from the nature of the triangle that the sum of its angles is equal to two right angles (I. prop. 17, schol.). They do not come out from him, but remain in him; just this fact