One of Kant’s innovations was a new analysis of the constitution of temporal experience. His famous theses about the role of synthesis in experience provide new insight into the paradoxes of temporal being or “becoming”, and its relation or non-relation to something outside of time. These had been raised by pre-Socratics like Heraclitus and Zeno of Elea, and more satisfactorily treated by Plato and Aristotle.
Heraclitus famously said that everything flows, you can’t step into the same river twice, and things change into their opposites. Zeno went in the opposite direction, conceiving space and time in terms of instants and points, neither of which have any magnitude. He then pointed out that motion at a durationless instant is a logical contradiction. On this basis, Zeno claimed to prove various things that violate common sense, such as that an arrow can’t fly, and that the speedy Achilles could never catch up with a turtle that had a head start. From this he concluded that motion, space, and time were mere illusions.
Plato seems to have at first focused on a sharp distinction between true “being” as eternal on the one hand, and becoming in time as mere appearance on the other. This distinction allowed him to have it both ways. But in dialogues that are thought to have been written later such as Theaetetus and The Sophist, he came to suggest that being and time are not simply two disjunct categories.
Aristotle made time and space more intelligible by developing notions of duration and extension. For Aristotle, duration and extension come first, while durationless instants, magnitudeless points, and pure flux are all abstractions. I see him as an early advocate of the primacy of process. For Aristotle, the key to making this viable is to be able to explain how becoming as we experience it is really not just a pure flux, but rather is full of islands of relative stability that allow us — contrary to Heraclitus — to reidentify objects as having an underlying basis of sameness that persists through various kinds of change. It turns out that the edges of the islands are not rigidly distinct, but he developed the notion conventionally translated as “substance” to explain our experience of the relatively persistent form of their middles.
It is here that Kant’s contribution is significant. Aristotle develops a plausible account of the persistence of form through change, but he discusses it mainly from the point of view of how things are, even though he separately suggests that experience is also shaped by processes of interpretation by us. Kant took up that suggestion, and developed it in considerable detail. Kant consistently emphasizes our role in constituting the stability of form of things we experience in time, though he also insists on an “empirical realism” that justifies most of what we get from so-called common sense. This implies that for Kant as well, there implicitly must be some basis in the way things are, for the stable constructs we come up with. Much of Hegel’s Phenomenology was devoted to a further development of these Kantian insights.
The neoplatonists and Augustine insisted that things in time have a source and destination in eternity. Classic neoplatonism attempted to treat this relation as a sort of quasi-logical unfolding of the divine essence, while Augustine identified it with the act of creation. The relation of temporal being to eternity remained a notorious point of difficulty in neoplatonism, while Augustine called it a mystery.
Hegel thought that Augustine ended up locating all reality in the Eternal, and that this resulted in a devaluation of actual life and experience. Aquinas already used ideas from Aristotle to allow for a more positive evaluation of temporal being. Some spiritual traditions go further and suggest that we humans have a sort of co-creator role in the world we experience. But it was Kant who mainly developed the basis for a non-supernatural explanation consonant with the spirit of this. The main point is that the world is not initially given in the form of pre-existing objects. We separate out objects from the sensible continuum, but at the same time this is not an arbitrary operation. We can’t just materialize a unicorn by thinking of one, but we do play a major active role in the construction of universals like “horse”, and in the recognition of persistent individuals.
Essences of things, once constituted, seem to “subsist” in some virtual way outside of time. The traditional view was that essences are straightforwardly built into the nature of things, or else simply dictated by God. Either way, this means that for us, they would be pre-given. I don’t think Aristotle really regards them this way, but only in the special case of biological organisms does he investigate their genealogy. Kant on the other hand effectively develops a generalized genealogy of essences, showing how they can be understood as temporally constituted.
Another of Kant’s big innovations is in explaining how we play a significant role in our own constitution. I think it is a grievous error to regard such processes of self-constitution as beginning with a blank slate, or as magically independent of real-world constraints, but there is a very important way in which we end up defining who we are — not by an explicit decision, but indirectly through the sum total of our commitments, actions, and responses to things.
That ethical “who we are”, while originating in time, is itself an essence with virtual subsistence. As with all essences, considered in its virtual subsistence, it is eternal. Aristotle would say that our essence stops evolving when our temporal being comes to an end. At that point, who we were is finally stabilized, as the total act of a life.