Our contemporary modal logic is usually considered as an extension of the two-valued predicate calculus which was developed in the late nineteenth century. However, as is commonly known, the roots of our modal theory reach far back to Aristotelian logic, which regarded modal logic as a legitimate branch of logical studies. Interest in modal notions is a new phenomenon among logicians only when it is considered in the framework of the developments of those late nineteenth century logicians who are honoured as the pioneers of modern logic.