The Science of Conjecture: Evidence and Probability Before Pascal

by James Franklin

Avicenna became known in the West chiefly as a medical writer; he and Galen were the most respected authorities until well into the sixteenth century. In this capacity he transmitted something of the ancient medical writers’ emphasis on reasoning from what happens “for the most part.” He notes the fallibility of induction, referring to Sextus’s example of the crocodile.34 But he suggests an answer that has considerable appeal: he expands a little the connection briefly found in Aristotle between reasoning from what happens “for the most part” and the exclusion of the hypothesis of chance: “Indeed, when we have the repeated sensation of [connection] between one thing and another, such as between purging and scammony, and between the observed movements and things in the heavens, this sensation repeats itself in our memory; following which, we acquire experience of it, the cause of it being a concomitant syllogism or memory: If this thing, for example purging relative to scammony, were due to chance, accidental, with no necessity in nature, it would not be possible in the majority of cases for the same to happen—so that, when it is not thus, the mind finds that fact extraordinary and looks for a reason . . . we have experienced this often and then reasoned that if it were not owing to the nature of scammony but only by chance, this would happen only on certain occasions.”35