The Logical Modalities

Among the many types of expression in our language which are used to qualify something, some have attracted more attention than others and become the subject of formal investigations. Most prominent among these are the qualifications ‘necessarily’, ‘possibly’, ‘contingently’, the so-called alethic modalities, which many philosophers since antiquity have felt to be of particular philosophical importance. Among other modalities whose formal properties have been studied are the epistemic ones (‘It is known that …’, ‘It is not known that …’, etc.), some moral modes, e.g., the modes of obligation, of the deontic modalities (‘It is obligatory that …’, ‘It is forbidden that …’, etc.). ¹ Many formal features recur from one type of these modalities to another. Professor G. H. von Wright, ² among others, has pointed out that many of these features are found also in quantification theory. Thus, e.g., ‘It is necessary that …’ behaves in many ways like the universal quantifier, and ‘It is possible that …’ like the existential quantifier. This has led von Wright to consider quantifications “existential modalities.”