It's defined too much by the code. What if I wanted to create a tree with a different slant? Or a taller one? What happens if the seed is in both sand and dust? All the elements we have right now are non-living - adding a tree wouldn't make it a particle simulator anymore.
However, I do think that everyone has their own idea of what TPT aims to be. This is an open-source project. Anyone can modify it - so everyone can have their own idea. For example, Ubuntu (which has user-friendliness in mind) was branched off of Debian (which has stability in mind).
My point was that the elements do not act in the way their living counterparts do. Does plant spontaneously appear the moment water touches it in real life? Does wood form out into branches when water is added in TPT? All that wood does is burn, which dead wood does. There is no concept of "life" in TPT, except for stickman, which is an exception, and doesn't really have anything to do with "life" except dying and regenerating health when plant is eaten. Even those are not replicas of real life.