 The key to the theory of argument, for Abelard, is found in inferentia, best rendered as ‘entailment’, since Abelard requires the connection between the propositions involved to be both necessary and relevant. That is, the conclusion—more exactly, the sense of the final statement—is required by the sense of the preceding statement(s), so that it cannot be otherwise. Abelard often speaks of the sense of the final statement being “contained” in the sense of the preceding statement(s), much as we speak of the conclusion being contained in the premisses. An entailment is complete (perfecta) when it holds in virtue of the logical form (complexio) of the propositions involved. By this, Abelard tells us, he means that the entailment holds under any uniform substitution in its terms, the criterion now associated with Bolzano. The traditional four figures and moods of the categorical syllogism derived from Aristotle, and the doctrine of the hypothetical syllogism derived from Boethius, are all instances of complete entailments, or as we should say, valid inference.