The Science of Conjecture: Evidence and Probability Before Pascal

by James Franklin

The allowable kinds of sign are therefore various and must be put together: “In general it is foolish to rely on a single sign: you will more likely have confidence in your conclusions when you find several signs pointing the same way.”4 This is indeed the main reason that a nondeductive science will look so different from a deductive one. Euclid does not collect different proofs for the same theorem, since the theorems are given the highest possible degree of belief by a single proof. But when conclusions are not susceptible of deductive proof, it will always be worthwhile to collect more “signs pointing the same way.” This means that wide experience is an advantage in sciences like physiognomy. One will need long experience to discern the difference between pallor due to terror and that due to fatigue and, generally, the congruity between different subtle shades of expression and conditions of mind.5 In such a science there must also arise problems when the signs do not point the same way, that is, when evidence conflicts: “It is safest, however, to refrain from all positive assertion when you find that your signs are inconsistent and contrary to one another in detail, unless they belong to classes, some of which you have determined to be more trustworthy than others.”