Modalities in Medieval Philosophy

Studies in modal notions, such as necessity, possibility or impossibility, have always played an important role in philosophical analysis. The history of these conceptions is the story of a variety of assumptions which have given shape to one part of rational discourse. A typical modern approach to modality is codified in what is generally known as "possible worlds" semantics. According to this, necessity refers to what is actual in any alternative state of affairs, possibility to what is actual in some, and impossibility to what is not actual in any alternative domain. The idea of spelling out the meaning of modal terms with the synchronic alternatives hardly occurred at all in ancient thinkers. They did not draw any sharp distinction between conceptual and real modalities and they were inclined to think that all generic possibilities must prove their mettle through actualization. Why and when did ancient modal conceptions and the modes of thought based on them lose their dominance?