These are the three fundamental ideas of his system.[122] The "Substance" of which he speaks is God, the infinite, self-existent, eternal Being, whose essential nature is defined in terms which might seem to be expressive of a great truth, for he says: "I understand by God an absolutely infinite Being, that is to say, a Substance constituted by an infinity of Attributes, each of which expresses an eternal and infinite essence." But, on closer inspection, we find that the God of whom he speaks is not the Creator and Governor of the world, not a living, personal Being, distinct from Nature and superior to it, not the Holy One and the Just, possessing infinite moral perfections, and exercising a supreme dominion over His works; but, simply, absolute Being, the necessary self-existent Substance, whose known "Attributes" are _extension_ and _thought_, and whose affections, or "Modes," comprehend all the varieties of finite existence; in short, it is Nature that is God, for every possible existence may be included under the twofold expression of _Natura naturans_ and _Natura naturata_.
Accordingly, the principle of _Unisubstancisme_ is broadly avowed, and the very possibility of creation denied.

He affirms, and, indeed, according to his definition, he is entitled to affirm, that there is not and cannot be more than _one substance_; for by "Substance" he means a self-existent, necessary, and eternal Being.

And, on the same ground, he affirms that the creation of _such_ a substance is impossible; for, having excluded every finite thing--everything that does not exist of itself--from his definition of Substance, he is warranted in saying that anything called into being by a creative act of Divine power could not be a "substance," _in his sense of that term_.

He sets himself to prove, by a series of propositions whose logical correctness, as deductions from his fundamental assumption, may be freely and most safely admitted, that the production of a "substance" is absolutely impossible; that between two "substances," having different "attributes," there is nothing in common; that where two things have nothing in common, the one cannot be the cause of the other; that two or more distinct things can only be discriminated from each other by the difference of the "attributes" or "affections" of their "substance;" and that, in the nature of things, there cannot be two or more substances of the same kind, or possessing the same attributes.

He holds, of course, that Nature is as necessary as God, or, rather, that God and Nature are one; there being but one Substance, appearing only in different aspects, as cause and effect, as substance and mode, as infinite and yet finite, as one and yet many, as ever the same and yet infinitely variable.
It is only necessary to add, that the sole attributes of this Substance which are capable of being known by our limited intelligence, and which are discerned by an immediate "intuition of reason," are two, namely, _extension_ and _thought_.