The Science of Conjecture: Evidence and Probability Before Pascal

by James Franklin

Whose business is it to explain the theory of probability? What discipline owns the theory and the right to evaluate applications of it in other fields? The modern answer has been mathematics. It is an answer that has brought a great deal of clarity into obscure matters and has, for example, saved millions of lives by finding accurate methods of testing drugs for their curative powers. Yet it biases the theory toward aspects of probability that can be given precise numbers. Dice are studied intensively; how to take into account context and background information is neglected. The ancient answer to the same question was that the theory of probability belonged to rhetoric. There are some advantages in that answer too, especially in directing attention to a wide body of data—all the arguments that are found to be persuasive in practice. The concomitant disadvantage is a persistent tendency to blur the distinction between what ought to persuade, because it is probable, and what can be made to persuade by giving it a color of plausibility. That is, the logical and the psychological are easily confused. It is true that the distinction is clearly stated by Aristotle and others and also that the ancient sense of “rhetorical” did not emphasize the merely persuasive as much as the modern sense does, but the focus on what actually persuades was enough to prevent an autonomous theory of nondemonstrative argument from being seriously developed in logic.