Creation Argument for the Existence of God
In Meditation III, Descartes gives a variation on his 'Trademark' argument for the existence of God
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there must be at least as much in the cause as there is in the effect" (p. 472)
The same is true for anything else that is less perfect than God.
With respect to his being uncreated and eternal, Descartes says that "it does not follow from the fact that I existed a short time ago that I must exist now" (p. 471). This is because "the entire span of one's life may can be divided into countless parts, each wholly independent of the rest" and "unless some cause, as it were, creates me all over again at this moment, that is to say, preserves me" (p. 471), it is not the case that I must exist now because I existed a short while ago.
This is because "the same force and action are needed to preserve anything at each individual moment that it lasts as would be required to create the same thing anew, were it not yet in existence. Thus conservation differs from creation solely by virtue of a distinction of reason" (p. 471)
Thus, in order to remain in existence from moment to moment, Descartes would need the same power that is required to create him.But he already knows that he does not have the power to create himself. So he knows that he does not have the power to conserve himself from moment to moment. So he knows that he is not uncreated and eternal, but that he relies upon the power of something else, something else which created him and which conserves him in existence.
The only possible creator of Descartes, then, is God. Only God can create a being that has an idea of God.