The ancient Greek philosophers were primarily concerned with the problem of change: how is it possible for the same thing to become different and yet remain the same thing? Things obviously change. But change seemed to require that one and the same thing could be opposite to itself, that it could be hot and cold, wet and dry, moving and at rest; yet such opposites were incompatible. Part of the answer to this difficulty may seem obvious, that in undergoing a change something that did not change took on incompatible opposites at different times. However, at first, the concept of time was not clearly separated from conceptions of change and motion. As with most early cosmological notions, time was first personified as a mythological deity and its cosmological significance was given in origin stories. Later, time came to be treated as an aspect of the natural world. Yet even the most sophisticated of Greek thinkers always treated problems about time as subsidiary aspects of problems about change and motion.