an affirmation is a positive assertion of something about something, a denial a negative assertion.

now it is possible both to affirm and to deny the presence of something which is present or of something which is not, and since these same affirmations and denials are possible with reference to those times which lie outside the present, it would be possible to contradict any affirmation or denial. thus it is plain that every affirmation has an opposite denial, and similarly every denial an opposite affirmation.

we will call such a pair of propositions a pair of contradictories. those positive and negative propositions are said to be contradictory which have the same subject and predicate. the identity of subject and of predicate must not be ‘equivocal’. indeed there are definitive qualifications besides this, which we make to meet the casuistries of sophists.