  Aristotle is also the originator of modal logic. In addition to quality (as affirmation or negation) and quantity (as singular, universal, particular, or indefinite), he takes categorical sentences to have a mode; this consists of the fact that the predicate is said to hold of the subject either actually or necessarily or possibly or contingently or impossibly. The latter four are expressed by modal operators that modify the predicate, e.g. ‘It is possible for A to hold of some B’; ‘A necessarily holds of every B’.