Hektoen International

A Journal of Medical Humanities

Nature telling her secrets: the Kepler–Descartes connection

Ronald Fishman
Chicago, Illinois, United States


Johannes Kepler

Nature tells us one secret in terms of another, and she may refuse to disclose one secret until another has been laid bare.

– T.S. Kuhn1

In 1604, Johannes Kepler solved the problem of how light is refracted within the eye to produce an image on the surface of the retina. This problem had confounded optical theorists before Kepler (including Leonardo da Vinci), who were stymied by the unavoidably inverted image produced by following refracted light rays through the transparent elements of the eye — they assumed such an inversion was incompatible with perceiving the world as right side up.2-4

Kepler, on the other hand, had made one crucial decision. As an astronomer, he decided to treat the eye simply as an optical instrument. He followed light within the eye to where it led him, and it led him to the retina. Yes, the image was inverted and reversed, but every point within it still bore the same relationship to the others as they did in the outer world. He then treated the problem of what happened to the retinal image afterwards as another problem, one whose solution would come later, by others. This is still a critical factor in determining the success of any scientific enterprise today. Kepler’s judgment here was as astute as any displayed in the astronomical discoveries for which he is famous.

Kepler’s image has “elegance,” that particular quality that Judson5 ascribes to the molecular structure of DNA: “Structure and function are united in DNA with such ingenious parsimony that one smiles with the delight of perceiving it.” In the eye, structure (the transparent media, the lens, the hemispheric retina) and function (the focusing of light on the retina) are also united with such ingenious parsimony that one smiles with the delight of perceiving it.

But at this point there was a problem. Kepler had taken what was then only a vague and tentative idea — that the retina was the true photoreceptor — and made it convincing because the image was a more or less accurate geometrical representation of space, its component points reproducing the organization of corresponding points in the external object. It thus was a powerful argument that however the mind or soul gained understanding from the image, its organization would be crucial.

Rene Descartes

However, Galen’s concept of the brain, still largely accepted in Kepler’s time, was of a mostly undifferentiated gland producing the animating principle, the “pneuma” or “spirit” of the nerves. What room was there in such a tradition for Kepler’s retinal image? Was Kepler’s representation of the outside world, so carefully constructed on the retina, to be lost in its passage to the brain? No, thought Rene Descartes, it was not lost. It was preserved in a direct projection of retinal points to corresponding points in the brain (Fig. 2).6-8 With one stroke, Descartes invented sensory projection and brought the physiology of sensation into the modern age. In all of the history of science there is no better example of how one man’s insight led directly to the equally powerful insight of another: for Descartes, Kepler’s image became the key to the brain.

Descartes proposed that the organization of the retinal image was maintained in passage to the brain by a mechanically feasible and internally consistent scheme. Each stimulated point on the retina exerted pressure on a fiber that ran back in the optic nerve past an uncrossed chiasm and ended on the internal surface of the ventricles. The interstices between the mass of fibers on that surface was considered a system of openings or pores. The sensory fiber controlled the relative orientation of a particular pore and the degree to which it was open, controlling the ease with which the intraventricular spirit or fluid emanating from a specific point on the pineal gland could enter a specific nerve, thus producing a conscious idea. The idea is accurate because there is a point-to-point correspondence between the patterns on the retina, the ventricle wall, and the pineal surface.9,10

Why the pineal gland? Descartes knew he must account for the experience of single binocular vision and for the single idea generated by paired sense organs stimulated simultaneously. Galen had already proposed the blood vessels around the pineal gland as the brain’s chief source of animal spirits. In a brain that seemed to be bilaterally symmetrical with paired structures, the pineal gland was conspicuous in that it was centrally located and unpaired. So Descartes selected it as the structure that allowed the “soul,” acting through the pineal gland, to obtain a unified sensation. This selection, sui generis in this context and unutterably quaint and wrong-headed to us, was logical to Descartes.11,12

From Descartes’ TREATISE ON MAN

Certainly this was very odd anatomy. It was, in fact, quite literally fantastic — a fantasy springing out of Descartes’ imagination. Voltaire called it a “novel of the soul.”13 But if we let the details of the anatomy distract us, we miss the larger point: Descartes’ single-minded, determined, relentless insistence on treating the brain as a complex mechanism. The pivotal points are these:

  1. The brain has an intricate structure.
  2. Important elements of this structure are on a level too minute to be seen, but are located in specific locations that have specific functions.
  3. The elements interact with each other in such a way as to form a model of the external world within the brain.
  4. This model then actuates the motor system in an organized way so as to deal with that external reality.
  5. Consciousness is closely related to this process.

The French Jesuit philosopher Nicolas Malebranche later wrote that when he first read Descartes, he was forced by palpitations of the heart to put the book down for a while, as he realized the implications of the brain as a maker and manipulator of symbols.2

It is true that we get impatient with Descartes for his “gratuitous precision” in constructing the body, describing things we know he could not have seen.11 (We do know that Descartes dissected animal parts, including sheep brains, obtained from his neighborhood butcher.) Then we realize that Descartes is being coy in his biological writings. He explicitly tells us that he is not actually describing the body but rather a machine that could simulate all the activities of the body. In effect he is proposing a hypothetical model, a technique still useful in modern science. He seems to realize that biology is not about to yield to his method of deducing laws of nature from a few basic axioms. Life is too diverse, too complex.

From Descartes’ TREATISE ON MAN

Descartes lived at a time when the established wisdom was Aristotle’s idea of the soul, and Aristotle’s soul, after all, at one time encompassed the whole body. It was the all-pervading activating feature of all body functions, of all life.14,15 Modern reductionist physiology — physical, chemical, molecular physiology — would have been impossible under it. Now Aristotle’s soul began to wither away. “There is a mask of theory over the whole face of nature,” said the nineteenth century philosopher of science William Whewell.16 Now the mask was changing. The terms of the problem were being redefined.

All these men faced a common problem when they thought about the nervous system. They recognized that it constituted a ramified network similar to that of the heart, arteries, and veins. Such a network meant something was transmitted or distributed. For the blood vessels, the blood was the obvious medium and the flow of a fluid could be readily understood. But no fluid was evident in the nerves except in the ventricles and enclosing sheaths (the reason why the ventricles had such prominence in these schemes). Hence the fluid must be “rarified”— too subtle to be visible. At the time this was a speculation based only on analogy with blood. Descartes’ novel proposition was to make the transmission similar to the bell-pull leading to servants in Victorian mansions — a physical continuity that acted as a valve controlling the flow of fluid. The corresponding points in the retinal image produced a symphony from such bell-pulls — a cerebral symphony. As a hypothetical model, this was a conceivable notion at a time when Galvani and his twitching frog legs were more than a century away. It was wrong, but when the time came, people were ready to accept biological electricity as the activating medium they were searching for.

All in all, Descartes gives us the impression that he would not be disconcerted in the least if one were to sit down with him in a quiet cafe with a bottle of good French wine and discuss neurons and synapses. When Descartes deals with the central nervous system, he has a recognizably modern sensibility.

The Cartesian model became part of the intellectual matrix of the time, usually unacknowledged or forgotten, wrong in the details, but still a program for thinking about the brain. The implicit promise was that if the brain could only be broken down into all its constituent parts, we would then understand how those parts worked together.

Modern knowledge of the brain has many roots. Kepler’s influence on Descartes is one of them, too significant to remain so unknown, and Descartes’ brain was a revolutionary idea. What we have been doing since then is changing the details. We recognize they were engaged in a search we are still bound on, for answers that still elude us.



  1. Kuhn TS. The Essential Tension: Selected Studies in Scientific Tradition and Change. Chicago: University of Chicago. 1977.
  2. Crombie AC: The mechanistic hypothesis and the scientific study of vision: some optical ideas as a background to the invention of the microscope. In: Bradbury S, Turner GL’E, Historical aspects of microscopy. Cambridge: Heffer; 1967:3–112.
  3. Lindberg DC: Theories of vision from al-kindi to Kepler. Chicago: University of Chicago; 1976:176.
  4. Casper M: Kepler. Hellman CD, trans, editors. New York: Dover Publications; 1993
  5. Judson HF: The Eighth Day of Creation: Makers of the Revolution in Biology. New York: Simon & Schuster; 1979.
  6. Pastore N: Selective History of Theories of Visual Perception. New York. Oxford University. 1971. 18-40.
  7. Polyak S: The Vertebrate Visual System. Chicago. University of Chicago. 1957. 100–5.
  8. Clarke E, Dewhurst K: An Illustrated History of Brain Function. Berkeley: University of California. 1972.
  9. Descartes R: Treatise On Man (original 1664)(trans: Hall TS).Cambridge: Harvard University. 1972.
  10. Descartes R: Discourse of Method: Optics, Geometry, and Meteorology (original 1637) (trans: Olscamp PJ). New York: Bobbs-Merrill; 1965.
  11. Hall TS: Descartes’ physiological method. J Hist Biol.1970.3:53–79.
  12. Scott JF: The Scientific Work of Rene Descartes (1596-1650). London: Taylor & Francis; 1952.
  13. Hatfield G: Descartes’ physiology and its relation to his psychology. In: Cottingham J, editor. The Cambridge Companion to Descartes. Cambridge: Cambridge University. 1992. 335–70.
  14. Clarke E: Aristotelian concepts of the form and function of the brain. Bull Hist Med. 1963. 37:1–14.
  15. Pagel W: The reaction to Aristotle in seventeenth century biological thought. In: Underwood EA, editor. Science, Medicine and History: Essays on the Evolution of Scientific Thought and Medical Practice New York: Oxford University. 1953. 489–509.
  16. Medawar PK: Pluto’s Republic. New York. Oxford University. 1982. 11



RONALD S. FISHMAN is a retired ophthalmologist. He has written over fifty publications, book chapters, and articles on the history of medicine.


Winter 2019  |  Sections  |  Science

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