I believe Descartes was afraid of water. Of course, water does not come up when Descartes mentions these “strange aversions” in his Passions of the Soul. He is careful. There are beasts, cats and worms, thunder, plants, roses and dead leaves. All belong to the earth. But water surges elsewhere when Descartes is genuinely at a loss. In the beginning of the second Meditation, for instance, when the evil genius has turned the whole world into an illusion, as well as the body, and mathematics. At this point, I haven't even proven my own existence. I haven't discovered that I think, so that I am. At this point, it is..

Comme si tout à coup j'étais tombé dans une eau très profonde, je suis tellement surpris que je ne peux ni assurer mes pieds, dans le fond, ni nager pour me soutenir au-dessous. (Meditations, 274)

Yes, as if I had fallen into deep water. And I can't feel the ground under my feet, and I can't swim.

But now that we think of it, let us come back to the first Meditation. It is not true that the evil genius has not turned the whole world into illusion. It has destroyed “the sky, the air, the earth, the colors and figures, the sounds and all the external things that we see” (Meditations, 272).

Imagine that we had been standing on the beach. If the evil genius took out “the sky, the air, and the earth,” he left the sea, which is now borderless. It is always like that, in the end: we are left with what we fear most.

Descartes believes that emptiness does not exist. The whole sky is filled with small particles pressed against one another. Maybe they could have been motionless. But God put them into motion. And what kind of motion can there be in the universe if it is full of matter? A translation somewhere requires another translation in the opposition direction, so as to fill the gap. So there can only be rotations, vortices. The whole sky is filled with whirlpools. It is like troubled water:

Pensons que la matière du ciel où sont les planètes tourne sans cesse en rond, ainsi qu'un tourbillon au centre duquel est le soleil […] Car tout de même que dans les détours des rivières où l'eau se replie en elle-même, et tournoyant ainsi fait des cercles, si quelques fétus ou autres corps fort légers flottent parmi cette eau, on peut voir qu'elle les emporte et les fait mouvoir en rond avec soi. (Principes, III, §30)

Indeed, the universe is like a whirlpool in a river, the kind of water in which one can't swim, nor feel the ground under one's feet. If you are afraid of emptiness, as Pascal was, if you have vertigo and see abysses opening up beside you, then you will imagine the world as an infinite void, where the earth is revolving without reason. But if you are afraid of water, like Descartes obviously was, then you will imagine the universe as a borderless sea full of whirlpools. That is the way our mind works.

The episode of the crossing of the Elbe is always invoked when the courage of philosophers is in question. The story is documented by Adrien Baillet, the first biographer of Descartes, in 1693. Descartes, who was then quite young, was traveling in Germany. There are various speculations as to the reason of this journey. But, anyhow, he took a boat across the river Elbe. The crossing could not have been long. After all, the Elbe is not a borderless sea. But, when the boat was in the middle of the water, Descartes noticed that the mariners were talking among themselves. Of course, on the ground, bandits may rob their client and let him run away, whereas on water they are stuck with their victim, unless they throw him overboard. That is the only way to get rid of him. Yes – thought Descartes – they are bandits.

Leurs délibérations allaient à l'assommer, à le jeter dans l'eau et à profiter de ses dépouilles. M. Descartes, voyant que c'était tout de bon, se leva tout d'un coup, changea de contenance, tira l'épée d'une fierté imprévue, leur parla en leur langue d'un ton qui les saisit, et les menaça de les percer sur l'heure, s'ils osaient lui faire insulte. [...] L'épouvante qu'ils en eurent fut suivie d'un étourdissement qui les empêcha de considérer leur avantage, et ils le conduisirent aussi paisiblement qu'il pût souhaiter. (Vie, 11)

The mariners indeed must have been surprised when they saw Descartes suddenly drawing out his sword and threatening them. I have my own interpretation of the episode. I believe the mariners were quite innocent. Descartes' tremor had nothing to do with them. It was the water. Consciously or unconsciously, he did just what, years later, in his Passions of the Soul, he would advise all of us to do who are plagued with these “strange aversions”: think of something that calls for bravery. Bandits. Descartes was not afraid of bandits. Besides, he had a sword while they were apparently unarmed. Thus, the ominous fear was transferred to another object, against which it was possible to fight, and feel brave.

I am not saying that Descartes deliberately decided to consider the mariners bandits. He may already have trained himself against his fear of water, and then it was all in his brain.