ERSES.

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.]

[Footnote 3: There can be no doubt that in their doctrine of
AEquipollents, the Schoolmen were trying to make plain a real
difficulty in interpretation, the interpretation of the force
of negatives. Their results would have been more obviously
useful if they had seen their way to generalising them.
Perhaps too they wasted their strength in applying it to the
artificial syllogistic