To put it another way, a thing or person is often seen as having a "defining essence" or a "core identity" that is unchanging, and describes what the thing or person really is. In this way of thinking, things and people are seen as fundamentally the same through time, with any changes being qualitative and secondary to their core identity (e.g. "Mark's hair has turned gray as he has gotten older, but he is still the same person"). But in Whitehead's cosmology, the only fundamentally existent things are discrete "occasions of experience" that overlap one another in time and space, and jointly make up the enduring person or thing. On the other hand, what ordinary thinking often regards as "the essence of a thing" or "the identity/core of a person" is an abstract generalization of what is regarded as that person or thing's most important or salient features across time. Identities do not define people, people define identities. Everything changes from moment to moment, and to think of anything as having an "enduring essence" misses the fact that "all things flow", though it is often a useful way of speaking.