The first principles of motion can be discovered but they cannot be demonstrated. In order to demonstrate them, we would have to assume that they are the result of other principles, in which case they would not be the first principles. These principles are not demonstrated but discovered by analyzing substantial changes. In his analysis of change, Aristotle discovered that every change implies duality. It implies a subject in potency which, by the action of some agent, pases into act, i.e. receives some new perfection or actuality.
Motion presupposes the acquisition of something and the corruption of something else.
The subject of change is what stays the same through the change. However, through the change, it acquires something new and loses what it previously had. Motion implies a passive principle and an active principle, intrinsic to the thing that changes. Thus, there are three principles necessary for change to take place. There must be something new that comes to be, something old that passes away, and something that stays the same throughout.