But now, if from the simple fact that I can draw from my thought the idea of anything it follows that all that I recognise clearly and distinctly to pertain to this thing pertains to it in reality, can I not draw from this an argument and a demonstration of the existence of God? It is certain that I do not find in me the less the idea of him, that is, of a being supremely perfect, than that of any figure or of any number whatever; and I do not know less clearly and distinctly that an actual and eternal existence belongs to his nature than I know that all that I can demonstrate of any figure or of any number belongs truly to the nature of that figure or that number: and accordingly, although all that I have concluded in the preceding meditations may not turn out to be true, the existence of God ought to pass in my mind as being at least as certain as I have up to this time regarded the truths of mathematics to be, which have to do only with numbers and figures: although, indeed, that might not seem at first to be perfectly evident, but might appear to have some appearance of sophistry. For being accustomed in all other things to make a distinction between existence and essence, I easily persuade myself that existence may perhaps be separated from the essence of God, and thus God might be conceived as not existent actually. But nevertheless, when I think more attentively, I find that existence can no more be separated from the essence of God than from the essence of a rectilinear triangle can be separated the equality of its three angles to two right angles, or, indeed, if you please, from the idea of a mountain the idea of a valley; so that there would be no less contradiction in conceiving of a God - that is, of a being supremely perfect, to whom existence was wanting, that is to say, to whom there was wanting any perfection - than in conceiving of a mountain which had no valley.

But although, in reality, I might not be able to conceive of a God without existence, no more than of a mountain without a valley, nevertheless, as from the simple fact that I conceive a mountain with a valley, it does not follow that there exists any mountain in the world, so likewise, although I conceive God as existent, it does not follow, it seems, from that, that God exists, for my thought does not impose any necessity on things; and as there is nothing to prevent my imagining a winged horse, although there is none which has wings, so I might, perhaps, be able to attribute existence to God, although there might not be any God which existed. So far from this being so, it is just here under the appearance of this objection that a sophism lies hid; for from the fact that I cannot conceive a mountain without a valley, it does not follow that there exists in the world any mountain or any valley, but solely that the mountain and the valley, whether they exist or not, are inseparable from one another; whereas from the fact alone that I cannot conceive God except as existent, it follows that existence is inseparable from him, and, consequently, that he exists in reality; not that my thought can make it to be so, or that it can impose any necessity upon things; but on the contrary the necessity which is in the thing itself, that is to say, the necessity of the existence of God, determines me to have this thought.

For it is not at my will to conceive of a God without existence, that is to say, a being supremely perfect without a supreme perfection, as it is at my will to conceive a horse with wings or without wings.

And it must not also be said here that it is necessarily true that I should affirm that God exists, after I have supposed him to possess all kinds of perfection, since existence is one of these, but that my first supposition is not necessary, no more than it is necessary to affirm that all figures of four sides may be inscribed in the circle, but that, supposing I had this thought, I should be constrained to admit that the rhombus can be inscribed there, since it is a figure of four sides, and thus I should be constrained to admit something false. One ought not, I say, to allege this; for although it may not be necessary that I should ever fall to thinking about God, nevertheless, when it happens that I think upon a being first and supreme, and draw, so to speak, the idea of him from the store-house of mind, it is necessary that I attribute to him every sort of perfection, although I may not go on to enumerate them all, and give attention to each one in particular. And this necessity is sufficient to bring it about (as soon as I recognise that I should next conclude that existence is a perfection) that this first and supreme being exists: while, just as it is not necessary that I ever imagine a triangle, but whenever I choose to consider a rectilinear figure, composed solely of three angles, it is absolutely necessary that I attribute to it all the things which serve for the conclusion that there three angles are not greater than two right angles, although, perhaps, I did not then consider this in particular.

Source: The Philosophy of Descartes in Extracts from His Writings. H. A. P. Torrey. New York, 1892. P. 161