Closely connected with such cases as the above are those mentioned by Archbishop Thomson as “Immediate Inferences by added Determinants” (Laws of Thought, Sec. 87).  He takes the case:  ’A negro is a fellow-creature:  therefore, A negro in suffering is a fellow-creature in suffering.’  This rests upon the principle that to increase the connotations of two terms by the same attribute or determinant does not affect the relationship of their denotations, since it must equally diminish (if at all) the denotations of both classes, by excluding the same individuals, if any want the given attribute.  But this principle is true only when the added attribute is not merely the same verbally, but has the same significance in qualifying both terms.  We cannot argue A mouse is an animal; therefore, A large mouse is a large animal; for ‘large’ is an attribute relative to the normal magnitude of the thing described.

Sec. 4.  Conversion is Immediate Inference by transposing the terms of a given proposition without altering its quality.  If the quantity is also unaltered, the inference is called ‘Simple Conversion’; but if the quantity is changed from universal to particular, it is called ‘Conversion by limitation’ or ‘per accidens.’ The given proposition is called the ‘convertend’; that which is derived from it, the ‘converse.’

Departing from the usual order of exposition, I have taken up Conversion next to Subalternation, because it is generally thought to rest upon the principle of Identity, and because it seems to be a good method to exhaust the forms that come only under Identity before going on to those that involve Contradiction and Excluded Middle.  Some, indeed, dispute the claims of Conversion to illustrate the principle of Identity; and if the sufficient statement of that principle be ‘A is A,’ it may be a question how Conversion or any other mode of inference can be referred to it.  But if we state it as above (chap. vi.  Sec. 3), that whatever is true in one form of words is true in any other, there is no difficulty in applying it to Conversion.

Thus, to take the simple conversion of I.,

    Some S is P; .’.  Some P is S.
    Some poets are business-like; .’.  Some business-like men are poets.

Here the convertend and the converse say the same thing, and this is true if that is.

We have, then, two cases of simple conversion:  of I. (as above) and of E. For E.: 

    No S is P; .’.  No P is S.
    No ruminants are carnivores; .’.  No carnivores are ruminants.

In converting I., the predicate (P) when taken as the new subject, being preindesignate, is treated as particular; and in converting E., the predicate (P), when taken as the new subject, is treated as universal, according to the rule in chap. v.  Sec. 1.

A. is the one case of conversion by limitation: 

    All S is P;
    .’.  Some P is S.