The system of Spinoza rests upon three fundamental conceptions, from which all the rest may be derived with mathematical necessity. These conceptions are that of substance, of attribute, and of mode.
1. Spinoza starts from the Cartesian conception of substance: substance is that which needs nothing other for its existence. But with such a conception there can exist only one single substance. A number of substances like that of Descartes is necessarily a contradiction. There can be nothing which has a substantial being besides the one substance of all things. This one substance Spinoza calls God. Of course, with such a view, the Christian idea of God, the notion of a spiritual and personal being, must be laid aside. Spinoza expressly declares, that his notion of God is entirely different from that of the Christian; he denied that understanding and will could be predicated of God; he ridiculed those who supposed that God worked for an end, and even scorned the view which regarded the world as a product of the Divine willing or thinking. God is, with him, only substance, and nothing more. The propositions that there is only one God, and that the substance of all things is only one, are with him identical.
What now peculiarly is this substance? What is positive being? This question it is very difficult to answer directly from the standpoint of Spinoza, partly because a definition, according to him, must contain (_i. e._ must be genetically) the immediate cause of that which is to be explained, but substance is uncreated and can have no cause besides itself; but prominently because Spinoza held that every determination is a negation, since it must indicate a want of existence, a relative not-being. (_Omnis determinatio est negatio_ is an expression which, though he uses it only occasionally, expresses the fundamental idea of his whole system.) Hence, by setting up any positive determinations of being, we only take away from substance its infinity and make it finite. When we therefore affirm any thing concerning it, we can only speak negatively, _e. g._ that it has no foreign cause, that it has no plurality, that it cannot be divided, etc. It is even reluctantly that Spinoza declares concerning it that it is one, for this predicate might readily be taken numerically, as implying that others, the many, stood over against it. Thus there can remain only such positive affirmations respecting it as express its absolute reference to itself. In this sense Spinoza says that substance is the cause of itself, _i. e._ its being concludes existence in itself. When Spinoza calls it eternal, it is only another expression for the same thought; for by eternity he understands existence itself, so far as it is conceived to follow from the definition of the thing, in a sense similar to that in which geometricians speak of the eternal properties of figures. Still farther he calls substance infinite, because the conception of infinity expressed to him the conception of true being, the absolute affirmation of existence. So also the expression, God is free, affirms nothing more than those already mentioned, viz., negatively, that every foreign restraint is excluded from him, and positively, that God is in harmony with himself, that his being corresponds to the laws of his essence.