Omne agit inquantum est in actu. Because the fullness and measure of act is operation, so far as something is perfect it is an agent exercising its agency. But interaction is only imperfect agency, for, qua interactive, an agent acts only as a part of a larger system of agency. And so in the measure that beings are more perfect, they rise above interaction (N.B. We are here only considering transitive actions).
On the lowest level one finds causes that only interact. These are purely physical entities. Because they only interact, they are only agents in an imperfect way, thus they only relate to final causes in an imperfect way, that is, while we can identify characteristic stages in a physical process, none of these stages is an absolute term. Masses do not, for example, seek some absolute natural place, nor is the change from iron to rust anything but a relative slice of an essentially infinite process, like the cycle of water evaporating, rising, condensing, and evaporating again. Because they only interact, large scale systems of physical causes do not arrange themselves in sharply divided causal hierarchies: they form a cloud system of mutual interactions. This does not preclude order from large scale physical systems, but it makes their order less like a watch and more like the weather.
On the highest level there are causes that only act and in no way interact. These are the purely equivocal causes where the agent is essentially separate from what he acts upon.
At the middle level, there are causes that in one way act and in another way interact. Such causes have a non-physical principle of action, though they also have physical existence as as part of their definition. These are the natural living things.