Changes in the Understanding of Possibility from Ancient Greece to Present

Ancient Greek philosophers mostly understood possibility in terms of the development of the cosmos. Something is possible if it is going to happen at a certain moment in time in the actual world. Modal concepts were thus closely connected with time: what is possible is sometimes actual, what is necessary is always actual, and what is impossible is never actual. Hintikka (1973) characteristically calls such interpretation, found in Aristotle’s physical and metaphysical works, as a statistical interpretation. An important consequence of this ‘temporal’ understanding of modality is the denial of mere possibilities: no genuine possibility can remain forever unrealized. Today the main point of possibility is exactly the fact that it is a simultaneous alternative to actuality, thus, unrealized. This comes out very clearly in the most intuitive interpretation of the possible worlds framework according to which one possible world within the set of all possible worlds represents the actual world, while all the rest represent the unrealized alternatives to it. How and when did this substantial change in the understanding of possibility, modality in general, come about? The need for a model with possibilities as simultaneous alternatives to reality was occasionally recognized already in Antiquity, especially in the discussions on human free will, given that the temporal understanding of possibility makes it very hard to reject determinism. It became even more pronounced in the Middle Ages when not only human but also divine free will had to be established. With the help of some examples, I will show how a new understanding of possibility slowly developed through the Middle Ages, and how Leibniz later utilized it in his
theodicy.