The Science of Conjecture: Evidence and Probability Before Pascal

by James Franklin

Kepler sees the divide between necessary and probable reasons for his theory as a result of introducing physics into astronomy. “As is customary in the physical sciences, I mingle the probable and the necessary and draw a plausible (probabilem) conclusion from the mixture. For since I have mingled celestial physics with astronomy in this work, no one should be surprised at a certain amount of conjecture. This is the nature of physics, of medicine, and of all the sciences which make use of other axioms besides the most certain evidence of the eyes. . . . Thus the physical difference is now immediately apparent [among the three opinions of Ptolemy, Copernicus, and Brahe]: by way of conjecture indeed, but yielding nothing in certainty to conjectures of doctors on physiology, or to any other natural science.”