ONE OF THE most fertile minds ever, Johannes Kepler (1571–1630) made valuable contributions to every field he addressed. He changed the face of astronomy by abandoning principles that had been in place for some 2,000 years, made important discoveries in optics and mathematics, and even constructed astrological charts renowned for their uncanny accuracy. In addition, he was an uncommonly good philosopher.
One tends not to hear much about Kepler's philosophical prowess, probably because he did not actually write a treatise specifically on philosophy; his philosophical views were usually advanced as solutions to problems in other disciplines (the Apologia pro Tychone contra Ursum is an exception, but its content was constrained by Tycho Brahe). Consequently, in order to encounter Kepler's philosophical views, one has to study his treatment of other subjects. Kepler's astronomical works provide a particularly fruitful source for his epistemology and methodology of natural philosophy, what we now call “science.”
For one thing, Kepler became a Copernican at a time when there was no empirical evidence that would support the Copernican over the wellentrenched Ptolemaic system (and, later, over the Tychonic system). His initial reasons for preferring Copernicanism, therefore, were extraempirical and were drawn in particular from metaphysics and methodology. Consequently, in his early astronomical works one finds interesting arguments on the logic of drawing predictions from theory, the virtue of the explanatory power of a theory, what makes a theory testable, and so on. Later, he added arguments from physics to his arsenal, which raised other philosophical issues.
For another thing, astronomers were well aware of certain philosophical problems having to do with the making and interpretation of observations. It was standard practice to process observations in the computation of planetary positions. One had to calculate the effects of refraction and to consider whether the body was observed at the horizon or directly overhead (consider the familiar phenomenon of the change in size of the moon as it approaches the horizon). One could not always make the desired observation at precisely the right time—for example, if it was clouded over. One had to take into account that, because the object stood at such a distance from the observer, a small error in observation could significantly influence the calculated position of the planet; and, if one was a Copernican, then the position of the Earth had to be taken into account. These and other considerations made it clear to astronomers that the relationship