Others (and this has some mediaeval usage in its favour, though not the most intelligent) would call the form All not-P is not-S (the Obverse or Permutation of No not-P is S), the Converse by Contraposition. This is to conform to an imaginary rule that in Conversion the Converse must be of the same Quality with the Convertend. But the essence of Conversion is the interchange of Subject and Predicate: the Quality is not in the definition except by a bungle: it is an accident. No not-P is S, and Some not-P is S are the forms used in Syllogism, and therefore specially named. Unless a form had a use, it was left unnamed, like the Subalternate forms of Syllogism: Nomen habent nullum: nec, si bene colligis, usum. TABLE OF CONTRAPOSITIVE CONVERSES. Con. Con. All S is P No not-P is S No S is P Some not-P is S Some S is not P Some not-P is S Some S is P None. When not-P is substituted for P, Some S is P becomes Some S is not not-P, and this form is inconvertible. OTHER FORMS OF IMMEDIATE INFERENCE. I have already spoken of the Immediate Inferences based on the rules of Contradictory and Contrary Opposition (see p. 145 - Part III, Ch. II). Another process was observed by Thomson, and named _Immediate Inference by Added Determinants_. If it is granted that "A negro is a fellow-creature," it follows that "A negro in suffering is a fellow-creature in suffering". But that this does not follow for every attribute[7] is manifest if you take another case:--"A tortoise is an animal: therefore, a fast tortoise is a fast animal". The form, indeed, holds in cases not worth specifying: and is a mere handle for quibbling. It could not be erected into a general rule unless it were true that whatever distinguishes a species within a class, will equally distinguish it in every class in which the first is included. MODAL CONSEQUENCE has also been named among the forms of Immediate Inference. By this is meant the inference of the lower degrees of certainty from the higher. Thus _must be_ is said to imply _may be_; and _None can be_ to imply _None is_. Dr. Bain includes also _Material Obversion_, the analogue of _Formal Obversion_ applied to a Subject. Thus Peace is beneficial to commerce, implies that War is injurious to commerce. Dr. Bain calls this Material Obversion because it cannot be practised safely without reference to the matter of the proposition. We shall recur to the subject in another chapter. [Footnote 1: I purposely chose disputable propositions to emphasise the fact that Formal Logic has no concern with the truth, but only with the interdependence of its propositions.] [Footnote 2: Mark Duncan, _Inst. Log._, ii. 5, 1612.]