Descartes' Proof for the Existence of God

Many readers follow Descartes with fascination and pleasure as he descends into the pit of skepticism in the first two Meditations, defeats the skeptics by finding the a version of the cogito, his nature, and that of bodies, only to find them selves baffled and repulsed when they come to his proof for the existence of God in Meditation III. In large measure this change of attitude results from a number of factors. One is that the proof is complicated in ways which the earlier discourse is not. Second is that the complications include the use of scholastic machinery for which the reader is generally quite unprepared -- including such doctrines as a Cartesian version of the Great Chain of Being, the Heirloom theory of causaltiy, and confusi ng terms such as "eminent," "objective" and "formal reality" used in technical ways which require explanation. Third, we live in an age which is largely skeptical of the whole enterprise of giving proofs for the existence of God. A puzzled student once remaked, "If it were possible to prove that God exists, what would one need faith for?" So, even those inclined to grant the truth of the conclusion of Descartes' proof are often skeptical about the process of reaching it.

Philosophers are inclined to evaluate arguments carefully. This page is aimed to help students analyze the complex elements of Descartes' proof into simpler parts and to provide some explanation of how those work, so that the student may grasp the nat ure of the proof and thus be in a much better position to give a reasoned evaluation of it. Such an exercise is both interesting and useful in itself, and also helps the student understand philosophical machinery which Descartes puts to other important u ses later in the Meditations.


At the beginning of Meditation III, Descartes has made some progress towards defeating skepticism. Using his methods of Doubt and Analysis he has systematically examined all his beliefs and set aside those which he could call into doubt until he reach ed one belief which he could not doubt -- that the evil genius seeking to deceive him could not deceive him into thinking that he did not exist when in fact he did exist. Having determined for certain that he exists, by a second application of the method s of Doubt and Analysis he has also determined that his essence is to be a thinking thing. And by yet a third application of these methods, he has also determined that the essence of matter (which can only be known by the mind) is to be flexible, changea ble and extended (if there is any such thing as matter). This is where things stand at the end of Meditation II.

At the beginning of Meditation III, Descartes makes yet more progress, he comes up with a criterion of certainty. By examining the truths which he discovered in the course of his second meditation, he decides that all of them have in common the proper ties of being clear and distinct. Thus, he claims "So, I now seem to be able to lay it down as a general rule that whatever I perceive very clearly and distinctly is true." He notes an objection to this claim, which is that he had previous ly accepted as evident things which turned out to be doubtful. For example he apprehended the earth, the sky and the stars with the senses, and saw these clearly and distinctly. He argues that what he in fact saw were the ideas of such things, and that he assumed without good reason that there were things in the external world which caused such ideas. This reflection again adds to the store of things which Descartes knows for certain, for now there are all of those ideas which clearly and distinctly appear before the mind. The only question is whether anything corresponds to and causes them.

If you find yourself puzzled about what the terms "clear and distinct" mean, you might want to engage in a search of the electronic text of the Meditations available on this web site to see if you can find passages which clarify the meaning of these terms. To do so go to Meditations and use the search engine available there

Again, considering even more simple and fundamental ideas, like those of mathematics, how could Descartes doubt that 2 + 3 = 5 or that he exists? The only way is by considering that God may have given him a nature which could be deceived about such th ings, or that God has changed the past. (Descartes has a quite extraordinary conception of Omnipotence -- even Artistotle had denied to the gods the power to change the past!) Even so, Descartes remarks:

Yet when I turn to the things themselves which I think I perceive , I am so convinced by them that I spontaneously cry out, let whoever can deceive me, he will never bring it about that I am nothing, so long as I continue to think I am something; or make it true at some future time that I never existed, since it is noe true that I exist; or bring it about that two or three added together are more or less than five, or anything of this kind in which I see a manifest contradiction.
--Cottingham Pg. 88
Descartes, can thus, hardly bring himself to doubt the truths of mathematics, any more than he can his own existence. And yet there is still a doubt.

So, while this source of doubt is very slight, and as Descartes' calls it metaphysical, it still needs to be removed in order to render the truths of mathematics or even the cogito-over-time, certain.

How is Descartes going to remove this metaphysical or hyperbolic doubt? The answer is that he is going to prove that God exists and is not a deceiver. Another way of putting this is that in order to get rid of the Evil Genius, Descartes must show tha t such a being could not exist. The only way of doing this effectively, Descartes holds, is to prove that another being, namely a God who is not a deceiver, does exist. If such a God exists, then an omnipotent Evil Genius is not possible.

So, how is Descartes going to prove that God exists? His means are limited. He know for certain that he himself exists and that his essence is to be a thinking thing. He knows for certain that the essence of matter (if it exists) is to be flexible, changeable and extended. He knows for certain that various ideas appear before his mind. So, basically he has himself as a non-extended thinking thing and his ideas with which to work. How can one fashion a proof for God's existence from these material s? That is Descartes' problem.

Determining the Kinds of Ideas and their relation to truth and falsity

We have now reached the beginning of the proof. We are now at ATVII 37 (Pg. 88 in Cottingham, and the last paragraph on Pg. 15 in Rubin's translation) Here is the text of our on-line translation:

5. Of my thoughts some are, as it were, images of things, and to these alone properly belongs the name IDEA; as when I think [ represent to my mind ] a man, a chimera, the sky, an angel or God. Others, again, have certain other forms; as when I will, fear, affirm, or deny, I always, indeed, apprehend something as the object of my thought, but I also embrace in thought something more than the representation of the object; and of this class of thoughts some are called volitions or affections, and others judgments.

6. Now, with respect to ideas, if these are considered only in themselves, and are not referred to any object beyond them, they cannot, properly speaking, be false; for, whether I imagine a goat or chimera, it is not less true that I imagine the one than the other. Nor need we fear that falsity may exist in the will or affections; for, although I may desire objects that are wrong, and even that never existed, it is still true that I desire them. There thus only remain our judgments, in which we must take diligent heed that we be nod deceived. But the chief and most ordinary error that arises in them consists in judging that the ideas which are in us are like or conformed to the things that are external to us; for assuredly, if we but considered the ideas themselves as certain modes of our thought (consciousness), without referring them to anything beyond, they would hardly afford any occasion of error.

7. But among these ideas, some appear to me to be innate, others adventitious, and others to be made by myself (factitious); for, as I have the power of conceiving what is called a thing, or a truth, or a thought, it seems to me that I hold this power from no other source than my own nature; but if I now hear a noise, if I see the sun, or if I feel heat, I have all along judged that these sensations proceeded from certain objects existing out of myself; and, in fine, it appears to me that sirens, hippogryphs, and the like, are inventions of my own mind. But I may even perhaps come to be of opinion that all my ideas are of the class which I call adventitious, or that they are all innate, or that they are all fictitious; for I have not yet clearly discovered their true origin.

Descartes begins by classifying the kinds of ideas he finds in his mind in order to determine which of them are proper bearers of truth and falsehood. Some are images of things, these are strictly speaking ideas, a man a chimera, the sky, an angel or God (Descartes seems to have forgotten at this point his own earlier example of the chilliagon, which he had used to show that some ideas are not images). Besides these there are volitions, emotions and judgements.

Considered in themselves, ideas are not false, for whether I am imagining a goat or a chimera, I am in both cases imagining them. Similarly, though I can desire something false or non-existant, I still desire them whether they exist or not. So, the o nly place where Descartes really has to worry about making mistakes is in making judgements. He continues: "And the chief and most common mistake which is to be found here consists in my judging that the ideas which are in me resemble, or conform to, thi ngs located outside me." Remember, Descartes had mistakenly held that the sky and stars existed, simply because the ideas of such things clearly and distinctly appeared before his mind

Descartes now classifies his ideas by their origin. Some ideas appear to be innate, others appear to be adventitious (that is to be caused by things outside of me), and still others to have been invented by me. My ideas of what a thing is, what truth is and what thought is seem to come from my own nature, i.e. to be innate. Hearing a noise, seeing the sun or feeling a fire, are ideas which come from things located outside of me. At least, this is what one would have thought bef ore plunging into the pit of skepticism! Finally sirens, hippogriphs and other such are my own invention. Still, while these three are the likely sources of all Descartes' ideas, he is not prepared at this point to say which ideas rightly belong in whic h category.

The class of ideas which are chiefly of interest here, are those which are derived from things located outside of me. So, Descartes askes himself what reason he has to think there are such ideas? Descartes has two reasons for thinking that ideas rese mble things. One is that Nature seems to have taught him this, and the other is many ideas appear in his mind independent of his will which implies that they do not depend simply upon him. Thus he feels the heat of the fire whether he wishes to or not, and this leads him to think that the idea is caused by something without him and "the most obvious judgement to make is that the thing in question transmitts to me its own likeness rather than something else." (Cottingham, Descartes, Selected Philosop hical Writings Pg. 89)

Descartes, however, dismisses these considerations as weak. Being taught by Nature, all this means is that "some spontaneous impulse leads me to believe it, not that its truth has been revealed by some natural light." So, it could be false. And Descartes is perfectly familar with ideas which are produced in him independently of his will -- namely dreams. So, these considerations are not enoough to persuade him that his ideas are produced by things located outside his mind. Even if such ideas did c ome from other things, it does not follow that the things must resemble the objects which caused them. Descartes has two ideas of the sun, one derived from the senses, the other from astronomy. The one derived from the senses makes the sun appear very s mall. On the other hand Descartes has an idea of the sun derived from astronomical reasoning which suggests that the sun is several times larger than the earth. Both of these ideas cannot be true. So, here is a clear case of an idea which, at least in ce rtain respects, does not resemble its cause. Descartes concludes that it is only blind impulse which "has made me believe up till now that there exist things distinct from myself which transmit to me ideas or images through the sense organs or in some ot her way." (Cottingham, Pg. 90)

He now comes up with a new way to investigate the question of the external causes of ideas. Here begins Descartes proof for the existence of God. And, as the proof begins we are immediately faced with some difficulties in understanding Descartes' ter minology.

More real?

Considered merely as a modes of thought, ideas do not differ from one another with regard to truth. This is because, so considered, it is true of all of them that they appear before the mind and seem to come in the same way. But insofar as different ideas are considered as images which represent different things, they differ widely. "Undoubtedly, the ideas which represent substances to me amount to something more and so to speak, contain within themselves more objective reality than th e ideas which represent modes or accidents." (Cottingham, Pg. 90) What does this sentence mean? In particular, what does Descartes mean by terms like, "substance," "mode" and "accident" and what does the phrase "objective reality" mean? How can something have more objective reality than somethething else?

In a footnote to the sentence quoted above, Cottingham adds this clarifies both the term 'objective' and 'formal'. 'Having more objective reality' means:

'...participation by representation in a higher degree of being or perfection.' (added in the French version). According to the Scholastic distinction invoked in the paragraphs that follow, the 'formal' reality of anything is its own intrinsic reality, while the 'objective' reality is a function of its represntational content. Thus if an idea A represents some object X which is F, then F-ness will be contained 'formally' in X but 'objectively' in A.
So, this is the explanation of 'objective' and 'formal' reality. Descartes uses yet another Scholastic term in regard to reality, that is 'eminent' reality. Something which is eminently real has more formal realtiy than something el se.

So why do ideas of substances have more formal reality than modes and accidents and thus give to the ideas which represent them more objective realtiy than modes or accidents give to the ideas which represent them? 'Substance' is another technical phi losophical term, rich in connotation. When Descartes talks about himself as a 'res cogitans' (a thinking thing) he is using the non-technical term 'thing' as a replacement for 'substance.' In fact, Descartes holds that he is a thinking substance. For D escartes, 'substance' means a thing which exists independently. 'Independent existence' is rather ambiguous. "Independent of what?" one migh ask. One answer is that substances exist independently of one another. Thus, were my piano destroyed, my kitche n sink might go on existing despite the destruction of the piano. 'Modes' and 'accidents' in contrast to substances are properties which depend on substances for their existence. If your green coat is burned to a cinder, the green of the coat cannot go on existing in spite of the destruction of the coat. So, modes and accidents are dependent of substances for their existence. So, one might say, that because of this dependence substances have more formal reality than modes and accidents.

Descartes goes on to claim that infinite substances have more reality than finite ones. And so if we put both of these points together we get:


(Descartes, a stone)
SUBSTANCESInfitite substances

Finite substances
ACCIDENTScolor (green)

Still, even though accidents and modes depend on substances for their existence, and even though infinite substances are greater than finite ones, why would Descartes hold that infinite substances are more real than finite ones and finite substances mo re real than modes and accidents?

The Great Chain of Being

Descartes use of Scholastic terminology is one clue. Descartes may well be thinking in terms of a doctrine called the Great Chain of Being which was familiar to his age and earlier, though not to ours. One assumpriion related to this doctrine is that to exist is to be good. So existence is a perfection. The greater the amout of being which something has, the better it is. Thus an infinite substance has more being and is thus more perfect and more real than a finite substance. Another principle re lated to the chain is sometimes called the Principle of Plenitude. It suggests that God, in his goodness, will create as much good as possible. So, if something can be created, God will create it. There is thus a chain of beings extending from God down through the tiers of angels to man and down through the animals to the lowest existence. Because of the Principle of Plenitude there are no gaps in the chain. If there were, God would have failed to create some good thing which he could have created. But, because of his supreme goodness, God would not do this. With the Great Chain of Being in mind, the doctrine that various species of substances have more formal reality than other species on the Chain of being makes considerable sense. And the idea of objective reality means that the ideas of those species represent the descending levels of formal reality of the Chain. Thus one idea will be more objectively real than another insofar as it represents a species of being higher on the Chain of Being than that other idea.

It has been suggested that Descartes's dualism, his distinction between mind and body, flatens the chain to just two links. But this is clearly not the case. There is, at the least, the distinction between ininite and finite substance; among finite s ubstances souls are presumably higher on the Chain than bodies; and then below bodies, one has modes and accidents. Even such an abbreviated Chain of Being is sufficient for the purposes of Descartes' proof for the existence of God.

Representation -- Am I alone?

Descartes next takes up the nature of representation. He is considering the possibility that though my idea has some objective reality, there is no need for the thing it represents to have the corresponding level of formal reality. His first point in rebutting this possibility is to note that "just as the objective mode of being belongs to ideas by their very nature, so the formal mode of being belongs to the causes of ideas -- or at least the first and most important ones, by their very nature." (Cottingham, Pg. 91) Descartes continues that though one idea may originate from another, an infinite regress is not possible here. At some point one must come to the archetype of the idea which must possess formally all of the perfection or reality which is present in the idea objectively. So Descartes says, it is clear by the natural light that while it is possible for an image or idea to fall short of the perfection of the things from which it is taken, it cannot contain anything greater or more perfect than its source. This reference to the natural light marks this claim both as one which Descartes claims to know for certain, and as an important step in the proof for the existence of God.

Descartes now goes on to explain how he is going to exploit this point in proving the eixstence of God. If it turns out that the objective reality of an idea is so great that he is sure that the same amount of formal or eminent reality cannot be found in him, then he himself cannot be the cause of that idea. It will then follow, as he says, "...that I am not alone in the world, but that some other thing which is the cause of this idea also exists." (Cottingham, Pg. 92) On the other hand, if there is no such idea -- that is one with a greater amount of objective reality than Descartes has formal or eminent reality, then Descartes will have no reason to hold that there is anything besides himself and his ideas.

Descartes reviews the ideas which he has, and finds that "...apart from ideas which give me some representation of myself...there are ideas which variously represent God, corporeal and inanimate things, angels, animals, and finally other men like myself." (Cottingham, Pg. 92) He realizes that his idea of angels could be put together from ideas he has of himself, of corporeal things and of God, even if there were no animals, angels or other men like himself. In respect to his ideas of corporeal things, none of them is so great as to contain more objective reality than Descartes could find in himself. So such ideas could have come from him. Examining his ideas of corporeal things (as he had the wax on the day before) he discovers that there are very few things which he knows clearly and distinctly about them. The list, he says, includes size, extension in breadth, length and depth, shape (which is a function of the boundaries of the extension), position (a relation between things possessing shape), and motion or change of position. To these he adds substance, duration and number. But the other kind of ideas, i.e. light and color, sounds and smells, taste, heat and cold and other tacticle qualities are so confused and obscure that "I do not know whether they are true or false." Descartes means that concerning color and smell, taste and so forth, he cannot tell whether the idea represent something really existing in the object or not. Such ideas, Descartes concludes, could definitely arise from him. As for the clear and distinct ideas, many of these -- especially such ideas as substance, duration and number, he could have borrowed from himself. He himself, that is, is a single substance which endures through time. As for such ideas as extension, shape, position and motion, which do not belong to Descartes as a thinking thing, he says that since he is a substance, these elements could be contained in him eminently. (Given the definition of eminent reality, one may puzzle over the meaning of this claim) (Cottingham Pg. 93)

Finally, Descartes turns to his idea of God to consider "whether there is anything in the idea which could not have originated from myself." (Cottingham, Pg. 93) Descartes continues:

By 'God', I understand, a substance which is infinite, independent, supremely intelligent, supremely powerful, and which created both myself and everything else (if else there be) that exists. All these attributes are such that, the more carefully I concentrate on them, the less possible it seems that they could have originated from me alone. So, from what has been said it must be concluded that God necessarily exists.

Here is a surprise -- "So, from what has been said, it must be concluded that God necessarily exists." This sounds like we are at the end of the proof, and we have just come to the conclusion that God necessarilty exists! In one sense this may be true, yet Descartes goes on for some time after this answering possible objections, clarifying points and so on.

Reconstructing the Argument

Still it might be as well at this point to try to reconstruct Descartes' proof for the existence of God using the materials which he says are enough to come to the conclusion that God necessarily exists. What is a reconstruction of an argument and how are we going to do this reconstruction. A reconstruction is like, but somewhat different than a summary of an argument. The aim of a reconstruction is to show all the parts of the argument and clearly express their relations to one another. A summary on the other hand often focuses on the most important elements and leaves out those parts which are not important. Reconstructions are more useful for the detailed evaluation of an arguement than a summary. A reconstruction of an argument has a format - - to list and number all the premises, then draw a line and put the main conclusion as the final proposition at the end. An intermediate conclusion -- that is a premise which is sufficently controversial to require argument for its truth -- can also be marked by drawing a line at the end of the list of premises which support that intermediate conclusion and then putting the intermediate conclusion. In making the reconstruction, one should begin with those claims which are explicitly given in the text, both premises and conclusions; and add only those implicit premises or conclusions which the logic of the argument requires in order to work. Implicit premises and conclusions (if any) should be distinguished by being put in square brackets [ ]. There are often a variety of ways in which an argument can be legitimately reconstructed. One constraint on reconstructions, however, is that one should follow the Principle of Charity. This principle sa ys that when given the choice between a variety of ways in which to take an argument, one should adopt the stronges interpretation, that is the one which is logically the strongest (both formally and informally) and which makes the premises most uncontroversially true. How detailed the reconstruction you make may vary considerably. In this case, you should also try to make as detailed a reconstruction of Descartes proof for the exitence of God as possible.

How should we begin to reconstruct Descartes' proof? The obvious thing to do is to start with the conclusion. So the first step in the reconstruction looks like this:

God necessarily exists.

So, there is the main conclusion. Now you need to find the premises.

Next we need to start finding the premises. Clearly, the proof depends on the fact that Descartes has an idea of God which has so much objective reality that it could not have been made by him. So, we are probably going to need the list of kinds of ideas. So:

  1. Ideas are either innate (inborn or known from one's own nature), adventitious (come from outside me) or made by me.
  2. Formal reality is characteristic of things.
  3. Some things have more formal reality than others.
  4. To exist is to be good.
  5. Greater goodness or perfection therefore implies that some things have more existence than others.
  6. Substances have a greater amount of formal reality than modes or accidents.
  7. Infinite substances have more formal reality than finite substances.
  8. Objective reality is the reality characterisic of ideas in virtue of the fact that the idea represents some realtiy.
  9. Some ideas have more objective reality than others, depending on the formal reality of the things which they represent.
  10. There is at least as much reality in an efficient cause as in its effect. (This is revealed by the natural light.)
  11. The ideas in me are like images that may well fall short of the things from which they derive but cannot contain anything greater or more perfect. (This is revealed by the natural light.)
  12. If I can be sure that the objective reality of one of my ideas is so great that it isn't in me either formally or eminently and hence that I cannot be the cause of that idea, I can infer that I am not alone in the world--that there exists something else that is the cause of the idea.
  13. I have the ideas of myself, of God, of angels, of animals, of physical objects and of other men like me.
  14. I could have composed my ideas of animals, other men and angels. (There is a brief argument on behalf of this premise.)
  15. I could have composed my ideas of physical objects without these existing. (There is an argument to show that this premise is true.)
  16. There is more reality in an infinite than in a finite substance.
  17. The more perfect serves as a standard to judge the less perfect.
  18. I use God as the standard to judge that I am imperfect.
  19. My grasp of the infinite must be prior to my grasp of the finite.
  20. The idea of God is completly clear and distinct and contains more objective reality than any other idea.
  21. But perhaps I am greater than I have assumed and so could be the cause of the idea of a being with all perfections.
  22. The gradual increase in my knowledge shows that I am imperfect. (All of these things are revealed by the light of nature)