The symmetrical character of natural activity conflicts with its need to get started. By the first, any action might is equivalent to its reaction – which does what is like trying to decide whether the curve is convex or concave. But getting started is a matter of the inert being actualized by the active, and this is not just convex-concave difference. The active an inert are asymmetrical and irreversible.
Objection: Take two bodies A and B that are twenty feet apart in empty space. They will act on each other gravitationally and get started moving toward one another. Any “conflict” seems so easy to overcome that its hard to see how it can get started.
Response: Let A = an unsuspended anvil 20 feet over a B-irthday cake. Everyone can see that this is a set of initial conditions we’re considering in media res. But the surprise of an anvil twenty feet off the ground is an insight into the fact that the no body has any reason to be where it is apart from the causal history that accounts for what its doing there. So the objections ends up showing the opposite: any initial condition reiterates the conflict in natural action between the requirement that it be interactive and thus symmetrical and and that it get started and therefore be asymmetrical.
Because of this, no natural being as such can act by itself but only by taking part in something else that acts by its own initiative and from motives that are its own.