There is a word coined by Aristotle which comes down the ages to us, bringing with it as it were the sound of the griding of the Spartan swords as they leapt from their scabbards on the morning of Thermopylae, the +enérgeia tês psychês+ the energy of the soul.

In this contrast of the potential with the actual, modern physics is turning to account the most familiar of Aristotelian distinctions that between dunamis and energeia. That kinetic energy appears to be imparted by impact is a fact of daily and hourly experience: we see bodies set in motion by bodies, already in motion, which seem to come in contact with them.