In philosophy, the term "modal" is often used to refer to modal logic. Traditional syllogistic logic focused, for the most part, on things which were certain and happened all of the time. This is the basis for most forms of mathematical logic. Modal expressions, such as "necessarily" or "probably" qualify statements, describing the way in which they are or are not true. Modal logic has been crucial to theology, and debates about the nature of free will. Although Aristotle in On Interpretation investigates the problem of future contingents, the main ancient school of modal logic was Stoic-Megaran. In the Middle Ages, a group of Scholastics called the Modistae, developed an important way of looking at how modal operators influenced systems of language and logic. There was a major revival of modal logics in the late 20th century especially focused on questions of possible worlds, counterfactuals, and ethics.