I'm using the word "soul" mostly because it's used so ubiquitous. However, I myself rather prefer the term "self". It doesn't come with the religious connotation, focuses only the individual itself and fully embraces the idea of mortality.
At the same time I think that, while not immortal, the thought part of the self holds powers beyond the limitations that are the mortal shell. Imagine it somewhat similar to how the body can produce offspring. But instead of binding this ongoing generation to the same plane, the mental part holds the full potential of possibility expanding far beyond physical limitation.
A strong will, in this sense, may be able to somewhere else, maybe an entirely different dimension, just by merely being freed (e.g. by loss of the outer shell, aka death), produce a spark of life. Essentially becoming like some sort of god creating an entirely real universe.