But how?
By proving that a "substance" cannot be produced.

And why may not "a substance" be produced?
Because, _by the definition_, "a substance" is that which is "self-existent." In other words, a self-existent substance cannot be created,--a truism which scarcely required the apparatus of a geometrical proof by means of propositions, scholia, and corollaries, or, as Professor Saisset says, with laconic naivete, "_ce qui a a peine besoin d'etre demontre_." But, while the only proof that is offered extends no further than to self-existent or uncreated substance, it is afterwards applied to everything that exists, so as to exclude the creation even of that which is _not_ self-existent; and this on the convenient assumption that whatever exists must be either a "substance," or an "attribute," or a "mode." And thus, partly by an ambiguity of language, partly by an arbitrary and gratuitous assumption, he excludes the possibility of Creation altogether.

Surely it might have occurred to him that by proving the necessary existence of an uncreated Being--a doctrine held by every Christian Theist--he did not advance one step towards the disproof of the possibility of creation, nor even towards the establishment of his favorite theory of _unisubstancisme_; for, grant that there is an uncreated and self-existent Being; grant, even, that there can be no more than _one_,--would it follow that there can be no created and dependent beings, or that they can only exist as "modes" or "affections" of that absolute Essence?
Might they not exist as _creatures_, as _products_, as _effects_, without partaking of the nature of their cause ?[126] Yet, if there be one idea more than another which Spinoza is anxious to extirpate, it is that of creation, and he summons the whole strength, both of his logic and sarcasm, when he has to deal with the argument from "final causes." And no marvel; for the doctrine of a creation would cut up his system by the roots.

The radical difference, in fact, between Theism and Pantheism mainly consists in this: that the former regards creation as distinct from the Creator, as the product of His omnipotent and free will, as the object of His constant providential care, as the subject of His supreme control and government; whereas the latter represents it as a necessary _emanation_ from the Divine substance, as an eternal _development_ of the uncreated Essence; the finite, in all its forms, being a "mode" of the infinite, and the temporary phases of nature so many transient but ever-renewed manifestations of the unchangeable and eternal.

These two conceptions are diametrically opposed; they cannot admit of conciliation or compromise; and hence the daring attempt of Spinoza to prove the impossibility of creation, even when he admits the existence of an Infinite and Eternal Being.
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