I used to picture two worlds, one physical and one spiritual, with traffic between them. I now think there is one world, built in levels. Particles make atoms, atoms make molecules, molecules make cells, cells make bodies, and bodies make minds. Each level is built from the one below and does things the one below cannot. That is emergence. Even space may be a level.
What feels spiritual is the high levels seen from inside. Three Realms makes that claim about the spiritual, and Transcendence makes it about the pull we feel toward it. Dennett saw Darwin’s idea as the bridge between mechanism and meaning. The bridge runs inside one world.
The ladder bends, though. By size, the cosmic web of superclusters sits at the top, under the edge of the observable universe. By organization, the brain rivals the web. One study found the two form networks of similar complexity, at scales 27 orders of magnitude apart.
What drives matter up the levels is selection, and not only in living things. Assembly theory measures how hard a thing is to build. A proposed law says a system gains functional information when selection favors what it does. Both are young and contested. Complexity seems to live at the edge of chaos, between frozen order and noise, and the brain may run there.
Then the ladder closes into a loop, in two ways. Joel Primack and Nancy Abrams adapt an idea of the physicist Sheldon Glashow. They draw the sizes of nature as a cosmic uroboros, a serpent swallowing its tail. Gravity rules the largest scales and, at the tiniest length physics can speak of, rules again, so the head may meet the tail. Roger Penrose draws a second loop in The Road to Reality: mathematics governs physics, physics gives rise to minds, and minds grasp mathematics. He treats mathematics as a world of its own, a question that stays open here.
Others count other worlds. Karl Popper named three: the material, the mental, and the cultural, meaning the theories and art that minds make. He held all three real and irreducible, and Dennett faulted that as dualism in disguise. On my reading all three are levels of one world, not separate places.
So the duality I started with dissolves. There is one world that folds back on itself, and we are the part of it that noticed.