The Science of Conjecture: Evidence and Probability Before Pascal

by James Franklin

The criticism Oresme is advancing is one that would now be described by saying that astrology is not structurally stable. If the genius of ideas is measured by the time taken for them to become common currency, then this one is a winner. It is almost totally original; although a few thirteenth-century writers had realized that there would be some problem if the ratios of the orbits were irrational,41 only Oresme understands what the problem really is. On the way to his result, Oresme, in 1350, has come up with the ideas of fractional powers and probability based on relative frequency, ideas that took three hundred years to reappear; structural stability did not appear again until Poincaré’s work of the 1890s.42 The idea is that a theory dealing with physical things must make predictions based on measurements, which do not have infinite accuracy. Therefore the theory must, at least usually, make similar predictions for sufficiently close initial conditions.