We require that the middle term, as used in one premise, should necessarily overlap the same term as used in the other, so as to furnish common ground for comparing the other terms.

Hence I have defined the middle term as 'that term common to both premises by means of which the other terms are compared.' (5) One at least of the premises must be affirmative; or, from two negative premises nothing can be inferred (in the given terms).
The fourth Canon required that the middle term should be given distributed, or in its whole extent, at least once, in order to afford sure ground of comparison for the others.

But that such comparison may be effected, something more is requisite; the relation of the other terms to the Middle must be of a certain character.

One at least of them must be, as to its extent or denotation, partially or wholly identified with the Middle; so that to that extent it may be known to bear to the other term, whatever relation we are told that so much of the Middle bears to that other term.

Now, identity of denotation can only be predicated in an affirmative proposition: one premise, then, must be affirmative.
If both premises are negative, we only know that both the other terms are partly or wholly excluded from the Middle, or are not identical with it in denotation: where they lie, then, in relation to one another we have no means of knowing.