@Reconstructo,
"Transcendental" is a modest euphemism for eternal. The transcendental is Eternity For Men, or the only eternity we humans, arguably, can know. And this eternity is
us. This eternity is the
essence of our minds. Flux flows over us, arguably, but this flux is never known in-itself. Our eternal or transcendental mind
structures this flux
automatically and constantly. Kant did what Plato could not do. He associated eternity with time.
Whereas for Plato, time was the emanation of eternity. (Is this correct?) Aristotle rejected this, and proposed something else. Eternity
does exist in time, but only to the degree that time is the recurrence of structures. Species of animals and plants, varieties of government. Time is Eternal only to the degree that certain forms subsist in matter which comes and goes. Why is the eternal important? Because truth is impossible unless what truth corresponds to
subsists. If the world changes essentially then all knowledge is only opinion, never truth. Or it's truth that spoils. I think the numen from the very beginning drove philosophy toward eternal non-relative truth. Parmenides is actually quite important. Already he joined the numinous and the transcendental. (By the way, I associate the numen with "transcendence," but numen feels like a safer word.)