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.