To conjecture about something is to measure its probability. The Art of Conjecturing or the Stochastic Art is therefore defined as the art of measuring as exactly as possible the probabilities of things so that in our judgments and actions we can always choose to follow that which seems to be better, more satisfactory, safer and more considered.
Ars Conjectandi, as quoted in Keith Devlin, The Unfinished Game, p. 106