For these reasons, as soon as my age permitted me to pass from under the control of my instructors, I entirely abandoned the study of letters, and re… - René Descartes

I entirely abandoned the study of letters. Resolving to seek no knowledge other than that of which could be found in myself or else in the great book of the world, I spent the rest of my youth traveling, visiting courts and armies, mixing with people of diverse temperaments and ranks, gathering various experiences, testing myself in the situations which fortune offered me, and at all times reflecting upon whatever came my way so as to derive some profit from it.

But immediately upon this I observed that, whilst I thus wished to think that all was false, it was absolutely necessary that I, who thus thought, should be somewhat; and as I observed that this truth, I think, therefore I am (COGITO ERGO SUM), was so certain and of such evidence that no ground of doubt, however extravagant, could be alleged by the sceptics capable of shaking it, I concluded that I might, without scruple, accept it as the first principle of the philosophy of which I was in search

Así como la existencia del yo ha sido, en el cogito, establecida por una intuición intelectual, también la existencia de Dios queda establecida en el argumento ontológico por medio de una deducción (que para Descartes es una serie de intuiciones intelectuales). La metafísica del cartesianismo y filosofías subsiguientes tienden, por modo inevitable, a demostrar las existencias, mediante actos intelectuales subjetivos.

Be that as it may, there is fixed in my mind a certain opinion of long [21] standing, namely that there exists a God who is able to do anything and by whom I, such as I am, have been created. How do I know that he did not bring it about that there is no earth at all, no heavens, no extended thing, no shape, no size, no place, and yet bringing it about that all these things appear to me to exist precisely as they do now?