It is certainly Descartes, or Cartesian philosophers, whom Pascal wishes to invite on the plank that stretches above the abyss. Descartes, whom Pascal could well call with a certain irony “the greatest philosopher in the world,” does believe that “there is no soul so weak that, being well conducted, cannot acquire an absolute power against its passions” (Passions, §50/721). Which, obviously, Pascal doubts: hence the test of the plank.

But it is not that Descartes ignores phobias. There are various passages in the Passions of the Soul that deal with absurd fears, what we would call phobias.

L'horreur est instituée de la nature pour représenter à l'âme une mort subite et inopinée en sorte que, bien que ce ne soit quelquefois que l'attouchement d'un vermisseau, ou le bruit d'une feuille tremblante, ou son ombre, qui fait avoir de l'horreur, on sent d'abord autant d'emotion que si un péril de mort très evident s'offrait aux sens, ce qui fait subitement naître l'agitation qui porte l'âme à employer toutes ses forces pour éviter un mal si présent. (Passions, §89, 737)

Descartes does not mention the same examples as Pascal but there is no doubt that the worm, the trembling leaf or the shadow are harmless even though they produce a deadly fear. The difference is that, according to Descartes, there are ways in which these fears may be overcome. In fact, Descartes has a complete theory about the mechanism and the origin of phobias, and a method for surmounting them.

Descartes does not use the term phobia but he does discuss “strange aversions” (Passions, §136, 758) or the “horror” which may produce the noise of a falling leaf. The principle that causes these aversions will also lead us to a method for overcoming our phobias: the brain keeps track of the association of our impressions. It is purely mechanical. The baby, in his cradle, is sleeping peacefully. The cat comes and mews aggressively. Then it turns away. But the baby has felt fright. And there it is, the path leading from the image of a cat to the feeling of fright is wired in the brain. Our man, now grown up, will have no trace of the scene in his mind. He remembers nothing, but his brain has been configured. The sight of the cat will automatically produce fright. Or the smell of roses will give him a headache.

L'odeur des roses peut avoir causé un grand mal de tête à un enfant lorsqu'il était au berceau, ou bien un chat le peut y avoir fort épouvanté, sans que personne y ait pris garde ni qu'il en ait eu après aucune mémoire, bien que l'idée de l'aversion qu'il avait alors pour ces roses ou pour ce chat demeure imprimée en son cerveau jusqu'à la fin de sa vie. (Passions, §136, 758)

It could even have happened when the baby was still in his mother's womb. A cat has startled the mother, and the baby sympathizes with his mother's emotion. His brain is wired. Each time he will see a cat, the spirits that circulate in his blood will be diverted towards the region that triggers fright. From there, some will go back into the body, and set into action various mechanisms – trembling of the hands, sweating, pallor. Some spirits will also hit the pineal gland in the middle of the brain and produce in the soul the feeling of fear. If they hit the gland hard enough, there will be no way to ignore the fear.

Et, comme l'âme, en se rendant fort attentive à quelque autre chose, peut s'empêcher d'ouïr un petit bruit ou de sentir une petite douleur, mais ne peut s'empêcher en même façon d'ouïr le tonnerre ou de sentir le feu qui brûle sous la main, ainsi elle peut aisément surmonter les moindres passions mais non pas les plus violentes et les plus fortes. (Passions, §46, 718)

Real fright even when it is triggered by something harmless like a house cat, is like thunder: it is not possible not to hear it. The best our man can do is to stop his legs from running. His trembling hands, his pallor, his sweat, or his tears are mechanical processes to which his soul has no access. But he can still control his legs. So the cat-fearing philosopher can only stand, trembling with fright, in front of the cat, his will bent upon the task of preventing his legs from running. And this because of something that he cannot remember, something that might have happened before he was even born.

This reference to events that happened during early childhood, or even before, is very different from psychoanalysis. For, according to Freud, it is not only that there is an unconscious: the problem is that the unconscious works. It relates images and expresses one with another. The little Hans is afraid that his father will devour him. But it is not possible to live in this way. Somehow the boy associates horses with his father and imagines horses will bite him. Now he is afraid of horses. It is not necessarily that he was scared by a horse earlier in his childhood. This fear expresses something else, which is worse. That is why the analyst must have the child talking, so that the real fear comes out. If the analyst only cured the child from the fear of horses, the real fear would just express itself in another way.

There is no such “work” of the unconscious in Descartes' text. The events of childhood, with the associations they had at that time, have just been wired in the brain. Descartes has not invented psychoanalysis but, I believe, he has invented behavioral therapy. For our man, trembling before his neighbor's cats, may be trained, so as to master his fear:

ceux mêmes qui ont les plus faibles âmes pourraient acquérir un empire très absolu sur toutes leurs passions si on employait assez d'industrie à les dresser et à les conduire. (Passions, §50, 721)

There is no need to talk about these “strange aversions.” No need to dig in the depth of an unconscious memory in order to uncover the scene, the event that shaped our fears. All we need is training. The principle that causes our phobias also enables us to get over them. Since the brain keeps track of the association of our impressions, we may work on associating the fearful image – the emptiness of the abyss, the claws of the cat – to a more pleasant, or less frightening, experience.

Is our man afraid of cats? His brain associates the image of a cat with danger and horror. That does not mean he is a coward. If he were confronted with humans, bandits, he would be brave. So let us present him with a kitten; he is a bit shaky but he still holds his position. Let him think of the beast as a bandit. Let him think about being brave: he draws out his sword. And we repeat the experience again and again, with bigger cats, until his brain associates the image of a cat with drawing out a sword and fighting back. Of course, in his brain, the impression of fear will still be related to the image of the cat. The connection cannot be erased but its effect may be weaken, because the spirits that animate the brain will divide and trigger both fear and bravery. The fear will be less strong. Instead of trembling and going all white, the man will face the cat with his sword in his hand. As Descartes remarks, even dogs may be trained:

Ainsi, lorsqu'un chien voit une perdrix, il est naturellement porté à courir vers elle, et lorsqu'il oit tirer un fusil, ce bruit l'incite naturellement à s'enfuir; mais néanmoins on dresse ordinairement les chiens couchants en telle sorte que la vue d'une perdrix fait qu'ils s'arrêtent, et que le bruit qu'ils oient après, lorsqu'on tire sur elle, fait qu'ils y accourent […] Puisqu'on peut, avec un peu d'industrie, changer les mouvements du cerveau dans les animaux dépourvus de raison, il est évident qu'on le peut encore mieux dans les hommes. (Passions, §50, 722)

Thus, to overcome our phobias, we must train, just like dogs. The problem with this kind of therapy is that even when, or if, it works, our phobias remain outside the domain of reason, or outside of what I may call my self. Pascal is right. The greatest philosopher in the world, on the plank, above the abyss, does not conquer his vertigo by reasoning with himself. “His imagination pervades,” as Pascal diagnoses. The Cartesian philosopher has just rewired his imagination, his brain in fact. However, he remains at the mercy of another of these strange aversions, which will leave him as powerless as the first time.

He was afraid of cats. Now he draws out his sword. He walks proudly into the world. But just when he steps on Pascal's plank, he realizes he also has vertigo: the ominous fear, the trembling, the pallor. All his training must be started again. This is no life.