XV
Some of the Stoics believe that air, being interchangeable with other elements such as fire and water, does not derive from without a fresh cause of fire; it kindles itself by its internal motion. Then in dissipating masses of thick, compact clouds it necessarily emits a loud noise from the bursting of such large bodies. Besides, the very conflict of the resisting clouds contributes to the energy of the fire. In the same way the hand contributes to the cutting power of an instrument, but the actual cutting is done by the steel.