Johannes Kepler and René Descartes differ on many levels in their philosophy and what they focused on primarily in their philosophies. Kepler and Descartes are both considered quite different in that Descartes was a clear rationalist that did not trust finding truths from sense, and Kepler largely found factual truths about the universe not from speculation but from observation and data. Although Kepler focuses far more on astronomy than Descartes in his philosophy, the fundamentals which constitute the way they find truths about the universe are quite different. On another hand, both of them use different methods, one rational and one empirical, to ultimately make claims about the existence and essence of God. Although most of Kepler’s discoveries about the universe were based in empiricism, his ideas about the final causal reasons for the harmonic motion of the planets is largely religious speculation and not necessarily based on observational data.
Kepler, in his many studies throughout his life, also published works regarding optics. He was concerned with the way in which light is refracted and affects the image of something when it reaches the eye, and was interested in the way in which the eye perceived the image from the light source. He was particularly focused on this because of the possible observational inaccuracies of celestial bodies which occur when light reflected from the planets are altered on its path and reception in the eye. He was able to deduce through observation and calculations that it was the retina that was the final acceptor of light, and was able to figure out the way in which light is reversed and inverted to represent the world in a way that is still a geometrically accurate representation of the objects in the world.
Descartes took this idea of the retina’s ability to accept light in a hyper specific way, and connected it to the brain and ultimately to the soul. It is in particular the pineal gland in the brain where the rational soul is found. There are several ways in which the pineal gland can be stimulated and cause a response, and then tend to be from sense organs according to Descartes. In this comparison, it is the sense of sight which is being focused on as Kepler’s discoveries on the photoreceptor qualities of the retina are evidently used by Descartes as the retina connects through a complex system to the pineal gland. It is the image received in the retina which is passed through the optic nerve, and ultimately controls the degree to which fluid of the spirit from the pineal gland will be released into the nerves and produce a thought. Descartes says that there is a strong connection between the activity of the retina when it picks up light and the pineal gland which responds to these stimuli accordingly. This is one of the few ways in Descartes philosophy that the mechanical function of the body connects with the God given rational soul as data from sense organs is sent to the pineal gland and gives the soul an opportunity to respond to the stimuli through the body in a rather mechanical fashion. With Kepler’s intricate understanding of the photoreceptor properties of the retina, Descartes was able to build from that the connection between the actual mind, or soul, with the mechanical nature of the corporeal world. Despite the very strong difference between the philosophical achievements of Kepler and Descartes, Kepler’s observation based discovery of the retina’s ability to create an image of the world is a crucial component to Descartes connection of the rational and the mechanical corporeal body.
Contrary to this example of the synergy between Kepler and Descartes philosophical and physiological discoveries, There are still many areas in which they differ greatly in their interpretations of the world. This is mainly due to the different approaches they have to attaining their respective ideas about the universe, where Descartes is more Cartesian and rationalist instead of largely based on observation like Kepler. A main contrast is in their approach to planetary motion, and what accounted for the existence of the planets and the stars. Descartes said that the universe was made up of vortices which govern the motion of the planets. He used this to account for the heliocentric nature of the universe, and claimed that the sun is at the center of these vortices, creating this sort of magnetic property that allows for the sun to be the center of the solar system and for the planets to move around the sun. He claims here that the universe is entirely made up of three substances, some which are luminous and constitute the center of the solar system as the sun, and opaque substance that create the planets which pass by transparent substances that make up all the space in between the luminous and opaque substances. He does not identify these planets as being in motion though, despite that this model is clearly heliocentric. He claims that the vortices around the sun will move the planets in their fixed position around the sun, but the planets like the Earth are not actually moving at the same time. This is because their position relative to other planets is fixed and unchanged according to Descartes. This model here also serves the purpose of helping clarify this heliocentric model, but not actually prove how the universe actually works and is largely speculation as Descartes does not believe in empiricism as a way to find truths. Differently, Kepler largely bases his understanding of planetary motion on extensive calculation of precise observation of said motions. He is able to deduce through this empirical approach to finding truths that each planet revolves around the sun in an elliptical and harmonious manner. He does not mention here that the planets are made of a particular substance separate to that of empty space and the sun itself, and is able to deduce these truths through observations and calculations, not speculation. Kepler also specifies that the planets are made of platonic solids which account largely for their interactions and motions as well. The main difference here is that Kepler’s model of the universe is not based on vortices like Descartes’, but through a similar magnetism and intricate planetary harmony that is proven through extensive observation.
Overall, it is the foundations which these philosophers use to find their truths about the universe and the natural world that make them so different. In certain instances, they can seem to be discovering or supporting very similar truths and ideas, but they often use very different approaches, with Descartes using a more Cartesian approach instead of empirical like Kepler to find truths.
Sources Used:
Kepler, Johannes. Astronomia Nova. 1609.
— Astronomiae Pars Optica, 1606.
— Mysterium Cosmographicum. Tübingen, 1597.