Cursus Philosophicus Thomisticus, Prima Pars Artis Logicae

The third operation of the intellect is carried out in reasoning, which necessarily requires inference and consequence, whereby one thing is deduced from another. Consequence in general is defined as an argumentation, that is, “an utterance in which, one thing being given, another follows.” Since every inference is grounded in the connection of one truth with another, when the term follows occurs in the definition of consequence, it does not mean “is connected.” If it did, only a good consequence would be defined, since only that has a connection and inference of one thing from another. But if follows is taken as common to good and bad consequence, it means “is signified to follow”: wherever utterances are joined by a sign of inference, there is a consequence, even if in reality the inference is not good and fitting.

What a good and bad consequence is—the first division of consequence in general—we explained in the preceding book, chapter 5. If someone asks why consequence is divided into good and bad, rather than true and false, I answer that consequence is not a proposition, to which truth and falsity alone pertain because it affirms or denies. Rather, it is an inferential connection of propositions, to which the proper arrangement and fittingness of the connection pertain. Fitting and unfitting, then, make a consequence good or bad, not true or false.

Consequence is divided secondly into material and formal. A material consequence is good only by reason of some matter; a formal consequence is good in any matter, provided its terms have the same form. Therefore, a formal consequence that is once good is always good. For example, if you say, “Some human being is rational; therefore every human being is rational,” this consequence holds in that matter, because it is necessary matter in which a universal can be inferred from a particular. But it does not hold by force of the form, since it fails in another matter of similar form: “Some human being is white; therefore every human being is white.” From this you will understand what the form of a consequence is, by reason of which it is said to be formally good. It is the arrangement of propositions and terms according to quantity, quality, and the other logical properties, ordered to inferring one thing from another. When one thing follows from another in any matter not by reason of the matter arranged, but by reason of the arrangement itself, the consequence is called formal. Similarity of form is the agreement of the arrangement of propositions in the same quantity, quality, and logical properties. We said above that logical properties include supposition, ampliation, and the like; they will be recognized more clearly from the delineation of the defects in consequences that we shall set out below.¹