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Descartes’ ontological argument for the existence of God

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The ontological argument is an argument which aims to prove the existence of God. It is said to be ontological, because it bases its proof on the definition of what the being (ontos) of God is: it is in the being of God to exist. Boethius (6th century) is generally considered to be the first to have proposed such an argument, but it is the formulation by Anselm of Canterbury in the 11th century that makes the argument famous. In modern times, the Cartesian version of the argument has been particularly influential, being the subject of several criticisms which will lead to rejecting the value of ontological arguments in general.

How the argument works

Although there are differences according to the authors, the structure of the ontological argument remains globally invariant.

  1. God is a perfect being.
  2. A perfection which did not include existence would obviously not be complete.
  3. So God is also endowed with existence.


Anselm of Canterbury’s argument, published in Proslogion in 1077, can be summarized as follows:

  • Proposition 1: God is something such that nothing can be thought of as greater;
  • Proposition 2: and it is quite certain that what is such that nothing can be thought of as greater cannot be only in the intellect;
  • Proposition 3: because if it is only in the intellect, we can think that it is also in reality, which is greater;
  • Proposition 4: but this is certainly impossible;
  • Proposition 5: there is therefore no doubt that there is something such that nothing can be thought of as greater, and that as much in the intellect as in reality.


René DescartesIt is admitted that Descartes took up Anselm’s argument in his work, but there is no source to support it with certainty.

The ontological argument appears in the three main presentations of his metaphysics: the fourth part of the Discourse on the Method (1637), Meditations on First Philosophy (1641), and Principles of Philosophy (1644).

Discourse on the Method, IV

“I was disposed straightway to search for other truths and when I had represented to myself the object of the geometers, which I conceived to be a continuous body or a space indefinitely extended in length, breadth, and height or depth, divisible into divers parts which admit of different figures and sizes, and of being moved or transposed in all manner of ways (for all this the geometers suppose to be in the object they contemplate), I went over some of their simplest demonstrations. And, in the first place, I observed, that the great certitude which by common consent is accorded to these demonstrations, is founded solely upon this, that they are clearly conceived in accordance with the rules I have already laid down In the next place, I perceived that there was nothing at all in these demonstrations which could assure me of the existence of their object: thus, for example, supposing a triangle to be given, I distinctly perceived that its three angles were necessarily equal to two right angles, but I did not on that account perceive anything which could assure me that any triangle existed: while, on the contrary, recurring to the examination of the idea of a Perfect Being, I found that the existence of the Being was comprised in the idea in the same way that the equality of its three angles to two right angles is comprised in the idea of a triangle, or as in the idea of a sphere, the equidistance of all points on its surface from the center, or even still more clearly; and that consequently it is at least as certain that God, who is this Perfect Being, is, or exists, as any demonstration of geometry can be.”

As in his future talks, the argument is based on an analogy with mathematical ideas: existence is in the idea of ​​a perfect being as the property of a geometric figure is in the idea of ​​this one .

Meditations on First Philosophy, V

The ontological argument appears in the 5th of the Meditations on First Philosophy. It can be summarized as follows:

  • [Premise 1]: everything that is clearly contained in the idea of ​​a thing, can be said with truth of this thing.
  • [Premise 2]: yet eternal and necessary existence is clearly contained in the idea of ​​God.
  • [Conclusion]: so it is true to say that God exists.

The basis of this argument lies in the principle according to which from knowing to being, the consequence is good. Since the Cartesian method privileges only clear and distinct ideas as a criterion of truth, it must in fact be agreed that everything that is obviously contained in the idea of ​​a thing is also necessarily found in this thing itself:

“But now, if just because I can draw the idea of something from my thought, it follows that all which I know clearly and distinctly as pertaining to this object does really belong to it, may I not derive from this an argument demonstrating the existence of God?”

For example, if the property of having its three angles equal to two right angles is clearly contained in the idea of ​​the triangle, it is legitimate to assert that the very nature of the triangle is to have its three angles equal to two right angles. Or, if I clearly see that the number 3 is odd, it is legitimate to assert this property about the number 3 itself (and not just its idea). It is not different with God, in whose idea eternal existence is clearly contained, as long as I pay sufficient attention to it:

“It is certain that I no less find the idea of God, that is to say, the idea of a supremely perfect Being, in me, than that of any figure or number whatever it is; and I do not know any less clearly and distinctly that an [actual and] eternal existence pertains to this nature than I know that all that which I am able to demonstrate of some figure or number truly pertains to the nature of this figure or number, and therefore, although all that I concluded in the preceding Meditations were found to be false, the existence of God would pass with me as at least as certain as I have ever held the truths of mathematics (which concern only numbers and figures) to be.”

Objections and rebuttals

The argument of the Meditations is most meticulous, because Descartes makes an objection against each stage of his reasoning (starting with the second premise, then the conclusion, finally the first premise):

  • [Premise 2]: existence is not necessarily enclosed in the idea of ​​God (I can conceive of him as non-existent);
  • [Conclusion]: even though existence would be in his idea, this does not prove that God himself would exist;
  • [Premise 1]: even though that would prove it, the idea of ​​God was an idea forged by me anyway, so it was only arbitrarily that I attributed all kinds of perfections to it, including the existence.

Each objection is immediately followed by its refutation.

[Premise 2]

“[Objection] This indeed is not at first manifest, since it would seem to present some appearance of being a sophism. For being accustomed in all other things to make a distinction between existence and essence, I easily persuade myself that the existence can be separated from the essence of God, and that we can thus conceive God as not actually existing. But, nevertheless, when I think of it with more attention, I clearly see that existence can no more be separated from the essence of God than can its having its three angles equal to two right angles be separated from the essence of a [rectilinear] triangle, or the idea of a mountain from the idea of a valley; and so there is not any less repugnance to our conceiving a God (that is, a Being supremely perfect) to whom existence is lacking (that is to say, to whom a certain perfection is lacking), than to conceive of a mountain which has no valley.”


“[Objection] But although I cannot really conceive of a God without existence any more than a mountain without a valley, still from the fact that I conceive of a mountain with a valley, it does not follow that there is such a mountain in the world; similarly although I conceive of God as possessing existence, it would seem that it does not follow that there is a God which exists; for my thought does not impose any necessity upon things, and just as I may imagine a winged horse, although no horse with wings exists, so I could perhaps attribute existence to God, although no God existed.”

We cannot conceive of God with attention other than as existing. Otherwise it would no longer be a question of God, who has the unique property of existing from all eternity. God therefore necessarily exists.

Also, those who claim that God does not exist do not clearly understand God. They forge in their minds a parody of the Supreme Being, which then allows them to deny its existence. If they knew the true God, who is perfect by nature, they would not be able to separate his eternal existence from him.

[Premise 1]

“[Objection] And we must not here object that it is in truth necessary for me to assert that God exists after having presupposed that He possesses every sort of perfection, since existence is one of these, but that as a matter of fact my original supposition was not necessary, just as it is not necessary to consider that all quadrilateral figures can be inscribed in the circle; for supposing I thought this, I should be constrained to admit that the rhombus might be inscribed in the circle since it is a quadrilateral figure, which, however, is manifestly false. [We must not, I say, make any such allegations because] although it is not necessary that I should at any time entertain the notion of God, nevertheless whenever it happens that I think of a first and a sovereign Being, and, so to speak, derive the idea of Him from the storehouse of my mind, it is necessary that I should attribute to Him every sort of perfection, although I do not get so far as to enumerate them all, or to apply my mind to each one in particular. And this necessity suffices to make me conclude (after having recognised that existence is a perfection) that this first and sovereign Being really exists; just as though it is not necessary for me ever to imagine any triangle, yet, whenever I wish to consider a rectilinear figure composed only of three angles, it is absolutely essential that I should attribute to it all those properties which serve to bring about the conclusion that its three angles are not greater than two right angles, even although I may not then be considering this point in particular. But when I consider which figures are capable of being inscribed in the circle, it is in no wise necessary that I should think that all quadrilateral figures are of this number; on the contrary, I cannot even pretend that this is the case, so long as I do not desire to accept anything which I cannot conceive clearly and distinctly. And in consequence there is a great difference between the false suppositions such as this, and the true ideas born within me, the first and principal of which is that of God.”

Descartes also asserts that there is in his mind this idea of ​​an infinite God. However, since his understanding is finite, he cannot be the author of this idea. This is also ontological proof, for it amounts to saying that if God exists as a concept, he must exist in reality, for such a concept could not otherwise be thought of since it is beyond our comprehension.

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