I'm a newbie here and this may be a very dumb question. I have absolutely no philosophical training, which may be very obvious. I may have trouble formulating my question. The question: If I have a thought, say, to move my left thumb how does that thought send off an electrical impulse to the muscles of my thumb? In other words what is the interface between thought and the initiation of an electrical signal down a motor nerve? You will probably say the thought itself is an chemical/electrical impulse but how is the thought initiated? What starts off the thought process? Sorry, I can hardly understand my own question. A corollary to this question is with myriad sensory imputs coming into our brains from our senses who or what is in the brain viewing all these inputs and making sense of it all? Is it just another part of our brains doing that?
I do not think the gist of your question can be answered conclusively with reference to a final cause (of thought and action) - or for that matter without appealing to some concepts/terms whose definition we must simply assume. I take the problem at issue here to regard understanding of one's self as a thinking and acting being - and how this understanding can than "move out into the world", again, through your acting in accordance with that very understanding.
You reflect upon the idea that you are a thinking and acting being, and then you express this idea directly, through moving your fingers, hence proving the direct relationship between your thinking and acting. So there is correspondence between the object of reflection and the object of expression, in this instance.
So I would say it is the possible of reflecting upon and expressing the same idea of yourself as a thinking and acting being, which enables a person to generate the kind of question you are asking.