The idea of Cause is a product of experience, is not a priori.

Problem of the first Cause.

Causing is but transmitting motion (or, alteration). The first Cause must be that which is immutable. Aristotle was right in the essence of his argument. But that which is immutable is Force. Now Force is not, as the speculators of the schools thought, a primary thing which acts and ceases, leaving the world to a force-less motion. Wherever there is motion there also is Force: motion is its material side, its shadow, its representation. Force is not even a thing that exists and ceases to exist to live as motion. Force is motion and motion is force. Force is power; motion is act. Force (Energy) being extra time and space, science and theology are both right one in declaring it immanent, for so, in a sense it is, the other in making it anterior to the world, in time and in space.

Act (Motion) is the manifestation of Power (Energy). But Power cannot, in being manifested, become other than itself (unless it contains in itself both contraries).

Power = Act.