The Worthlessness of the Cartesian Proof – Étienne Gilson

August 1, 2012


To the Most Wise and Illustrious the Dean and Doctors of the Sacred Faculty of Theology in Paris.
We have faith that the human soul does not perish with the body, and that God exists, but it certainly does not seem possible to persuade infidels of any religion, or of any moral virtue, unless, to begin with, we prove these two facts by natural reason.

Étienne Gilson (1884-1978) was a renowned French philosopher and historian of philosophy, and a member of the prestigious French Academy. He was a prominent leader in the 20th century resurgence of the philosophy of St. Thomas Aquinas.


After passing twenty centuries as the very model of those self-evident facts that only a madman would ever dream of doubting, the existence of the external world finally received its metaphysical demonstration from Descartes. Yet no sooner had he demonstrated the existence of the external world than his disciples realized that, not only was his proof worthless, but the very principles which made such a demonstration necessary at the same time rendered the attempted proof impossible.

Descartes had first postulated that all self-evident knowledge arises from thought, and from thought alone. From this it follows that the existence of the external world cannot be considered immediately evident, but Descartes hoped to demonstrate the existence of the external world by applying the principle of causality to our sensations.

Like everything else, sensations must have a cause. Now, we are not conscious of being their cause; rather, we undergo sensations. Nor are we conscious of receiving them directly from God; on the contrary, we feel that we receive them from beings external to our thought. Since we have no clear and distinct idea that would authorize us to regard God as the cause of our sensations and, on the contrary, have a very strong natural inclination to regard them as caused within us by certain other beings, we must affirm that those beings do exist. For God is perfect and therefore unable to deceive us, but he would be deceiving us if he himself were to give us such ideas directly, all the while allowing us to be controlled by our irresistible natural tendency to believe that sensations come from something outside of us. Therefore, it has been proven that the external world exists.

FootNote: See R. Descartes, Discours de la methode, commented upon by E. Gilson (Paris: J. Vrin, 1925), 358-59, concerning moral certitude, which is not, however, an immediate metaphysical proof of the existence of the external world. Concerning the Cartesian demonstration itself, see E. Gilson, Etudes sur le role de la pensee medievale dans la formation du systeme cartesien (Paris: J. Vrin, 1930), 234-35.

It may be worth mentioning here that Descartes himself affirms that his proof of the external world is based upon causality: “It should be noted that this axiom must necessarily be admitted since it alone is the foundation of our knowledge of all things, both sensible as well as those which cannot be grasped by the senses. For how else, for example, do we know that heaven exists?” (Descartes, Secondes reponses, ed. Adam Tannery, 9:128).

In opposition to this interpretation, which an analysis of the proof itself confirms abundantly, some have tried to maintain that Descartes proves the existence of the external world by means of the divine veracity. This is an obvious distortion which will not suffice to place in doubt the nature of what Descartes did and said that he did. Certainly, divine veracity enters into the proof, but only to prove that the external cause of our sensations is not God. Descartes had foreseen Berkeley and in this way sought to exclude his position. Far from eliminating the proof by causality, the fact of God’s veracity actually renders it possible. The divine veracity in effect legitimizes this particular application of the principle of causality, thus permitting him to affirm that the external cause of sensations is indeed the material world of extended bodies.

As for the historical consequences of the Cartesian proof of the existence of the external world, see E. Gilson, The Unity of Philosophical Experience (New York: Scribner’s, 1937) chap. 2, “The Cartesian Experiment”, 125-220.

Thus reduced to its essentials, this demonstration has three main parts.

  1. First, an analysis of sensation which makes it appear, in contrast with images, as a fact independent of the will and imposed upon thought from without.
  2. Second, an appeal to the principle of causality which permits us to posit, beyond thought, a cause of our sensations.
  3. Third, an appeal to the divine veracity to assure us that the true cause of sensations is in fact the existence of created beings distinct from thought, and not God.

By proceeding in this manner, Descartes has afforded us a perfect example of a doctrine in which the existence of the external world is arrived at as the result of a deductive proof, using thought itself as a starting point. This is, as was later said, an “illationism”, a name which may be applied to any doctrine which proves the existence of the external world by applying the principle of causality to a particular content of thought.

FootNote: This is why it is impossible to deny that the doctrine of Cardinal Mercier constitutes, on this very point, an illationism of the Cartesian variety (see E. Gilson, Le Realisme methodique [Paris: Tequi, n.d.], 18-32). The arguments advanced to distinguish his position from Descartes’ are rather curious.

It is said that Cardinal Mercier does not base his proof, as did Descartes, upon divine veracity. Nobody claims that he did. Let us say, first of all, with Descartes himself, that the Cartesian proof is based upon the principle of causality, as is Cardinal Mercier’s. In order to distinguish the two on this point it would be necessary to prove the contrary.

But the most remarkable argument consists of maintaining that Cardinal Mercier merely developed various illationist arguments in passing without incorporating them into his doctrine and, we are assured, without abandoning the immediatism which he had professed from the first. It seems we must choose between making him an immediatist, an illationist or a babbler. I, for one, believe that he was a very coherent illationist. Msgr. L. Noel prefers to maintain, at the same time, the illationism of the Cardinal, which he can hardly deny, and the persistence of his original immediatism, concerning which he adds: “How, then, can we say that his thought is coherent?

Here we are reduced to hypotheses, perhaps not entirely satisfactory, which leave us in a certain amount of confusion which it may be impossible entirely to avoid” (“Les Progres de 1′epistemologie thomiste“, in Revue neoscolastique de philosophie 34 [1932]: 430). In other words, he admits to having contradicted himself in order to have one chance in two of being correct. But we need not become involved in such a discussion, for if it is true, and Msgr. Noel assures us it is, that Cardinal Mercier always admitted “a proof of the external world based upon the principle of causality” (art. cit., 431), then the rest of his thought must be interpreted as a function of this constant.

This may not be as difficult as we have been told it would be. The texts cited by Msgr. L. Noel (Notes d’epistemologie thomiste [Paris and Louvain, 1925], 221-23) are not at all opposed to illationism, for they affirm:

1) the existence of an internal reality, starting point for this illationism;

2) a knowledge of the passivity of our sensations, which will authorize as with Descartes, a search for the cause outside the sensing subject;

3) the fact that the mind, from the outset, represents all that it grasps in nature as existing in itself, which, for Cardinal Mercier, serves to prove that we have a self-evident certitude of the existence of substance.

The texts from Criteriologie cited by Msgr. Noel in “Les progres de 1′epistemologie thomiste” (432, n. 2) present no more problems than the first group of texts. In this group the Cardinal affirms two ideas that he always maintained at the same time and that are not in the least bit contradictory:

1) “We have a direct sensible intuition of external things”; direct, in that we first perceive actual things rather than the fact that we do perceive them;

2) “But it is impossible for us to affirm with certainty the existence of one or many extramental realities without making use of the principle of causality.”

The basic idea throughout seems to be that the act by which we directly, and without any reflexive intermediary, receive perceptions of the real as real nevertheless does not guarantee any certitude as to the extramental existence of this reality. I must therefore insist that Cardinal Mercier held to a perfectly coherent form of illationism and that he was in agreement with Descartes in that they both considered it necessary to prove the existence of the external world. This could be done by starting from the passive character of sensation and then completing the proof with the aid of the principle of causality. This is the extent of my thesis. It cannot be refuted by disproving what I did not include in it.

Whatever one may think of Descartes’ proof, it has this merit: it openly relies upon a deductive process. And it must, since it regards as insufficient our natural feeling that the existence of the beings apprehended in virtue of the union of body and soul is self-evident. Instead, it relies upon a special operation of the understanding to confer an intellectual certitude upon our natural feeling, guaranteeing it by means of the principle of causality.

The flaw in this doctrine is not in the reasoning itself, which is impeccable, but in the fact that Descartes was unable to explain sensation without admitting the substantial union of body and soul. Now, although Descartes himself did not realize it, such a union is incompatible with his demand for their complete and real distinction. [My emphasis – DJ] As a result, although they also started with a thought which is thought alone, Regius, Geraud de Cordemoy, Malebranche and, generally speaking, those called “Cartesians” quickly arrived at the conclusion that sensation does not imply any action of the body upon thought. [On this subject, see the very clear and well-documented study by H. Gouhier, La Vocation de Malebranche (Paris: J. Vrin, 1926), chap. 3, "Le Principe des cartesiens", 80-307.]

From this it follows that no content of sensation can serve the principle of causality as a starting point from which the existence of the external world may be deduced. Indeed, it was precisely because it was necessary for them to prove the existence of bodies that their proof was impossible. But it mattered little to them. If they could not prove that the external world exists, they believed it through faith in revelation.

Then came Berkeley, who simply observed that nothing in the Genesis story was changed whether one accepted or denied the existence of matter. He then concluded, and quite logically, that, if it is neither possible to know nor necessary to believe that the external world exists, the wisest thing to say is simply that matter does not exist. [For a short but more detailed study of this historical problem, see E. Gilson, The Unity of Philosophical Experience (New York: Scribner's, 1937), chap. 6, 376-97]

When Thomas Reid recounted this remarkable story [Thomas Reid, Oeuvres completes, published by T. Jouffroy (Paris, 1828), 3:148-223; "Essay on the Intellectual Faculties of Man", essay 6, chap. 2: "On Common Sense".] he was one of the first to discern its meaning, and it was his intent to escape the magic circle in which philosophers since Descartes had been trapped, mesmerized by the cogito and idealism without ever managing to get out.

It was in large measure his resolute rejection of the Cartesian approach that led Reid to elaborate his doctrine of “common sense”. Reid never pretended that he had discovered common sense, but he tried to give the expression, which itself had become common, a technical philosophical meaning.

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One comment

  1. Gilson recounts a very exciting story—it’s a historical—analytical sketch, engaged and dashing, with that special aplomb of this thinker. Thus, there are two streams—Reid plus his followers, and the neo—scholastics, two distinct tendencies. The metaphysicians who strove to rescue or at least shape a scholastic notion of the common sense—Zigliara, Sanseverino, Liberatore—are all third—rate authors, anyway, below Reid and his disciple Jouffroy.

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