Abstract
Essential properties are usually thought as properties that things must always possess, whereas accidental properties are considered as changeable. In this paper, we challenge this traditional view. We argue that in some important cases, such as social or biological development, we face not only the change of accidents, but also the change of essences. To analyze this kind of change we propose an alternative view on the relations between the modalities and time. Some properties might be necessary or possible for a thing in a classical sense throughout its existence, whereas others might be necessary or possible only for some restricted periods. We distinguish therefore absolute, prospective, retrospective, and relative modalities. As we argue, these non-classical concepts of modality are useful in analysis of some puzzling case of seemingly changing essences.