The Science of Conjecture: Evidence and Probability Before Pascal

by James Franklin

Pascal’s most successful experimental work in science was his investigation of pressure in fluids. His debate on his results with the Scholastic Père Noël provided the occasion also for considerations on the philosophy of science, which raised many of the issues still current in that subject. They include the role of deduction versus probability in the evaluation of alternative hypotheses, the significance of crucial experiments, the place of metaphysics in science, and the degree to which hypotheses can survive adjustments to fit the experimental facts. At issue was the explanation of experiments in which inverted glass tubes of water or mercury of sufficient height were observed to have spaces under the glass at the top of the tube. Were these essentially vacuums, as Pascal thought, or had some subtle matter passed through the glass to fill the space? And was the correct explanation of the phenomenon that of Pascal, according to which the pressure of the outside air could support a column of a certain height but no higher, or the Scholastics’ maxim that “nature abhors a vacuum”?