The Summary of Physics Restored

194Reason establishes that common fire too belongs to the class of spiritual rather than bodily things. If it were bodily, it would have a body of its own, inseparable from itself, no less than earth, water, air and the other perceptible natures. These consist in and are bounded by their own bodies; in and through those bodies they exist, exercise their powers and present themselves to sense. Fire, however, has no perceptible body of its own and resides only in another’s. A coal is not fire but burning wood, nor is flame fire but kindled smoke. In a word, this robber always feeds upon what belongs to others. It lives on plunder and is extinguished when that fails, having nothing within itself on which to feed. Moreover, a body added to a body increases quantity, but fire added to wood or smoke does not do so. Neither wood nor smoke is increased in quantity by the addition of fire. It therefore becomes clear that a spirit, rather than a fiery body, invades wood or smoke. A sword melted while its sheath remains untouched, and bones broken while the flesh remains uninjured by the fiery bolt of a hurled thunderbolt, sufficiently show that even lightning’s fire has a spiritual nature. Yet we must admit that fire is not wholly immaterial. It consists of exceedingly fine matter, through which it adheres to the surrounding air, and hence can be held by coarser matter. It therefore deserves the name of spirit rather than body, because it lacks perceptible quantity and cannot be apprehended unless it puts on another body.