Posterior Analytics (Bouchier)/Book I/Chapter XXX

Chapter XXX: On fortuitous occurrences

 * No demonstration can prove fortuitous circumstances, for demonstration deals only with the necessary or sometimes with the probable.

No knowledge of a fortuitous occurrence can be attained by demonstration. The fortuitous does not resemble either the necessary or the probable, but is that which falls outside both of these classes, while demonstration deals with one or other of them, since every syllogism is drawn from necessary or probable premises. If then the premises be necessary the conclusion is so likewise, if the premises apply, in most cases only, the conclusion has a similar application. Hence, if the fortuitous be neither probable nor necessary, it cannot be demonstrated.