"Ultimate reality" means whatever exists at the most basic level. Thus, particles are more basic than macroscopic objects but are composed of energy which therefore is yet more basic.
One view was that discrete atoms and/or souls, which could have existed independently, happen to coexist and to interact. Thus, atoms combine as material bodies in some of which souls are incarnated. Alternatively, several "elements" accidentally coexist and interact. A subtler understanding was that opposites are not only interactive but also interdependent.
A modern understanding is (something like) that a single field of energy variously and transiently manifests itself as particles, larger structures and consciousness. Thus, an individual subject of consciousness is not an independent, immaterial, immortal soul but a transient, material organism generating consciousness by sensitively interacting with its environment. Buddhists got this idea but retained "rebirth" as a hangover from the strong Indian traditional belief in reincarnating souls.
Thus, as Poul Anderson wrote in the SFWA Bulletin, concepts of ultimately reality change, although maybe not constantly.