Once we have decided, we then act, an external form of interaction. At no point here have we specified any 'thing' that was performing these processes. Another important aspect of our human interactions is that we learn by them. Our mental world is not a fixed object but an ever changing set of memories, abilities and desires. But other systems change with time also, they do not need to be conscious.
Rivers develop whirlpools, ecosystems evolve.
These structures are not imposed from outside, they are created by the internal interactions within the system. This we call self-organisation and it is a feature studied extensively in complexity science. Such studies take into account how animals and plants develop, how brains learn, how societies and organizations structure themselves and look to identifying the conditions under which such processes occur. Brain, Bodies and Environment Systems in general occur in many forms, those of interest to us here are those with sufficient complexity to produce interesting properties. Whilst we often think of our brain as the only intelligent part about us, this is not in general true.