It is a generally recognized truth that physical events can be looked at in two ways: from the mechanistic and from the energic standpoint. The mechanistic view is purely causal; it conceives an event as the effect of a cause, in the sense that unchanging substances change their relations to one another according to fixed laws.
The energic point of view on the other hand is in essence final; the event is traced back from effect to cause on the assumption that some kind of energy underlies the changes in phenomena, that it maintains itself as a constant throughout these changes and finally leads to entropy, a condition of general equilibrium. The flow of energy has a definite direction (goal) in that it follows the gradient of potential in a way that cannot be reversed. The idea of energy is not that of a substance moved in space; it is a concept abstracted from relations of movement. The concept, therefore, is founded not on the substances themselves but on their relations, whereas the moving substance itself is the basis of the mechanistic view.