Erfahrung und Vorurteil im naturwissenschaftlichen Denken Johannes Keplers

The change from ancient and medieval to modern natural science, called Wende (instead of ‘revolution’), must be associated with the work of Johannes Kepler and not that of Nicolaus Copernicus. Copernicus merely showed the way, introducing heliocentricity as the order of the planets. This Wende resulted from the synthesis of several disciplines formerly isolated from each other, namely mathematical (i.e. hypothetical) astronomy, new physics, mathematical harmony, astrology, new physical optics, and natural theology. Whereas Copernicus united mathematical astronomy and peripatetic (Aristotelian) physics, Kepler was first to see the necessity for providing a physical explanation and an ontological foundation to the heliocentric system. He was the first to consider and measure the movement of the planets in depth. The elements for his new physics Kepler obtained not from newly observed data, but from a harmonic archetypus of the regular polyhedra fitted in between excentric planetary spheres. On the basis of this archetypus (which he considered to be God's model in creating the universe) he accepted the new heliocentric planetary system as a physical reality. That is why astronomy, by way of taking into account stereometric quantities, is, in Kepler's eyes, a kind of divine worship. Later, the best empirical data had also to be taken into consideration as a means of proving this a priori archetypus (Vorurteil, preconception). The result was, on the one hand, a universal natural science able to explain natural processes in grater abundance than ever before or since in the history of science. Although accepted only in parts, it resulted in founding a new natural science with adherent mathematical and empirical methods. It also led Kepler to establish, step by step, the elliptical path of the planets, thereby overcoming, for the first time, the two axioms of ancient astronomy, requiring uniform and circular planetary motion. It has been shown that this Keplerian Wende was possible only within the Historischen Erfahrungsraum (‘historical field of experience’) of Renaissance Humanism (cf. this Journal 9/1986, p. 201), which came about itself as the result of reactivating the scientific and philosophical thinking of the ancient Greeks and was accomplished by three steps (phases) relating to the revival of (1) original ancient writings, (2) the ancient knowledge of natural facts and data, and (3) the ancient scientific and philosophical ideas and mentalities (Drei-Phasen-Modell).