Three are the main principles of Physics according to Aristotle,
a. Change and development
b. Cause of this change
c. And mechanism
Change and movement may be the meaning of kinesis, concerns any kind of change of matter, including the change of place, phora, and even alteration (= alloiosis), the movement namely in relation to the quality of matter.
What, however, causes this change of objects?
A pair of opposites is what causes a change, the cold and heat, but each one cannot activate directly to the other. Before the matter becomes hot and cold there is a stage between. Aristotle formulates the distinction between the potential and the actual. And he often calls attention to tow different forms, which shows the opposition between the potential, and the actual.
A child has the potential to become a man, the potential is what leads to the actual and the actual cannot be upturned. And as far as the exercise of knowledge it concerns, the potential plays a significant role. A man for example who has not studied low, but he chose to defend alone himself, can be characterized as potentially lower.
This distinction, namely, is found during the final state of a body, which has no other power to develop, and it passes to its actual state, and this is its kinesis.