Aquinas’s First Way seeks to demonstrate the existence of God from the undeniable fact of motion (e.g., the bringing to be of some potency O into actuality P). Although, the First Way does not necessarily demonstrate the existence of the Trinitarian God of the Bible, Aquinas’s argument for the existence of God should not, nevertheless, be considered or confused with an attempt at doing so. Simply, the Prima Via is an a posteriori argument that proceeds from empirical evidence which seeks to demonstrate a general conception of God and His attributes. To return, attempts at describing the nature of this argument and process of movement should then begin with the Aristotelian principles of potency and act.
Below you will find my attempt at doing so. Please forgive me for any errors, as I am not a philosopher. I am simply a Seminarian who is fascinated with the thought of Thomas Aquinas.
That said, potency can be described as the possibility that a thing might have (e.g., the possibility of Socrates becoming a grammarian). Likewise, actuality might then be described as the fulfillment of that possibility (e.g., Socrates actually becoming a grammarian). For some potentiality to receive the principle of act, the potentiality must be acted upon by a thing that already exists in a state of act, as a potentiality cannot do anything of itself qua potentiality.
Now with respect to motion, what these principles describe is a process in which a thing that is in motion receives its act from another. That is, what is in motion (i.e., a thing’s transition from potentiality to act) must receive its movement from another mover, already in the state of act (i.e., the thing which imparts the principle of act upon some potency thus brings into being the fulfillment of some latent possibility inherent to the thing considered).
To end, motion might then be simply described as the process of the movement of a thing from potentiality into actuality. That is, the undergoing of a thing’s state of potentiality to act from its previous state of being. For what exists in a state of potentiality can do nothing of itself except by being acted upon by some extrinsic or intrinsic thing that already exists in a state of act.