In Walden, Henry David Thoreau wrote,
Time is but the stream I go a-fishing in. I drink at it; but while I drink I see the sandy bottom and detect how shallow it is. Its thin current slides away, but eternity remains.
Thoreau is using the ancient distinction between time and eternity, along with the metaphor of time as flow or movement. In order for time to be seen as movement or a flow, there must be a nonmoving substratum or background (the riverbank or riverbed) which we can call eternity. But these aren’t realities that exist independent of an observer. This is a semantics that reflects the structures of a particular society.
The stationary societies of the old world had described themselves as objects, using such concepts as being, essence, nature, and genre. Within this structural and semantic framework, the possibilities of evolution were not excluded; but their observation and description could remain on the surface and work with the graphic concept of motion, which as counterconcept presupposes something fixed, like the banks of a river. (Theory of Society, vol. 2, 344)
Rather seeing time as flow or movement over a stationary background, we can see time as the difference between past and future. There is a continuous differentiation between past and future.
The Aristotelian unmoved mover is another way of envisioning eternity. But for a functionally differentiated society, the old semantics doesn’t work very well. Social structures have changed; therefore, semantics must ultimately change, although we often observe a semantic lag. For instance, when people speak of evolution in the sense of getting better or improving, we see a semantic lag. Evolution is about differentiation, not improvement or progression; it’s not about movement. In evolution, there are no points of origin and ending, no alpha and omega. There is no linear narrative.
It must be emphasized that differentiation does not mean a difference in essence. Essence as a concept belongs to the “old semantics,” or the semantics of a stratified or centralized society. Essence is the basis of classical ontology But in modern society, one social system is not simply different than another social system. A system differentiates out of society, or “out-differentiates.” Society, in other words, becomes different from itself. A social system becomes different from itself. Similarly, as as a person changes, she becomes different from herself. There is no movement toward perfection or completion, but just continual differentiation from oneself.