ABSTRACT
Traditional logic recognised as valid two forms of hypothetical argument that belong not to the logic of terms but to the logic of propositions. These were called by medieval logicians the modus ponens and the modus tollens. The logic of propositions consists of those laws of logical necessity which hold between propositions whatever their internal structure may be. These laws can be presented in a logical vocabulary which consists simply of signs for unanalysed propositions, for negation, for the conjunction and disjunction of propositions and for the relation of antecedent and consequent. What determines the truth of compound propositions, that is to say propositions which are negated or involve disjunction, conjunction, material implication or material equivalence, is the truth or falsity of the uncompounded propositions from which they are constructed.
