It's so wild that you mention that; just yesterday @TheLynx and I were at a book store and I happened to be skimming some Kant, and flipped right to the section on noumena/phenomena.
The funny thing is, we really don't know anything outside of our brain & it's filters/imaginings. I've had conversations and spent a good deal of time with many folks who, as far as I can tell, are experiencing completely different realities from the one I'm experiencing.
All true from the perch I sit on. We're like leaves fallen from a tree and carried by the wind. There is not much use trying to fight the flow.
The thing is though that now there are nasty machines loud and noisy that blow us every which way; machines that cut our trees down too soon; machines that shoot holes in us as we fall to the ground.
My focus isn't so much about who's right and who's wrong when it comes to metaphysics; my focus is arguing for the continuing overarching structure of secularism as it gives the most amount of freedom to the most amount of people in their quest for what's really real about reality. In that sense, I'm a secular spiritualist.
I would say that Daoism is a fine example of secular spirituality. Although a religious form did develop later.