Abstract

The reason for Aristotle’s treatment of (traditional) fourth figure syllogisms as first figure syllogisms with inverted terms in the conclusion is the following: To disprove the conclusiveness of a premiss pair Aristotle formulates two triplets of true propositions such that two of them correspond to the premiss pair in question and that the third proposition corresponding to a conclusion is an a-proposition in the first case, an e-proposition in the other. Since the truth of an a-proposition grants the falsity of the contrary e- and of the contradictory o-proposition, the first triplet offers two counter-instances for invalid syllogisms with true premisses and false conclusions. Similarly the true e-proposition grants the falsity of an a- and an i-conclusion. Since an a-proposition can be converted to an i-proposition and an e-proposition is equivalent to its converse, these first figure triplets also disprove any first figure syllogism with converted conclusions, with the exception of o-conclusions. The invalidity of the latter ones, however, can be shown by using premiss conversions of (invalid) second and third figure syllogisms. The proposed explanation also makes clear why there are no rejection proofs for invalid syllogisms of (traditional) fourth figure syllogisms in the Analytics.