ABSTRACT
A careful analysis of some passages of the Prior Analytics shows that according to Aristotle a syllogism should not only be valid, namely, having its conclusion resulting necessarily from its premisses, but also enjoy the trait of sharing validity with all inferences of the same structure, or “form”. Aristotle’s conception of the form of an inference is, however, different from the modern one because it is not based on the idea of substitution in a common schema. It is based instead on the idea of a common structure of semantic relations between terms, where the semantic relations can be expressed by means of very different formulations (for instance, “every” and “all” can be used to express the same semantic relation between terms).
