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A reason proves a thesis when it satisfies three conditions: it is present in the subject, it is present in all similar cases, and it is absent from all dissimilar ones. Smoke on the hill proves fire because smoke is on the hill, wherever there is smoke there is fire, and where there is no fire there is no smoke. The study of reasons classifies the correct signs, by effect, by nature and by non-observation, and catalogues the ways a reason goes wrong.
The pages here will present the three modes, the kinds of correct reason, and the fallacies.