We now come to a principle which conveniently sums up these conditions; it is called the _Dictum de omni et nullo_, and may be stated thus: Whatever is predicated (affirmatively or negatively) of a term distributed, With which term another term can be (partly or wholly) identified, May be predicated in like manner (affirmatively or negatively) of the latter term (or part of it).
Thus stated (nearly as by Whately in the introduction to his _Logic_) the _Dictum_ follows line by line the course of a Syllogism in the First Figure (see chap.

X.Sec.

2).

To return to our former example: _All authors are vain_ is the same as--Vanity is predicated of all authors; _Cicero is an author_ is the same as--Cicero is identified as an author; therefore _Cicero is vain_, or--Vanity may be predicated of Cicero.

The _Dictum_ then requires: (1) three propositions; (2) three terms; (3) that the middle term be distributed; (4) that one premise be affirmative, since only by an affirmative proposition can one term be identified with another; (5) that if one premise be negative the conclusion shall be so too, since whatever is predicated of the middle term is predicated _in like manner_ of the minor.
Thus far, then, the _Dictum_ is wholly analytic or verbal, expressing no more than is implied in the definitions of 'Syllogism' and 'Middle Term'; since (as we have seen) all the General Canons (except the third, which is a still more general condition of formal proof) are derivable from those definitions.