For everything which is of a warm and fiery character is agitated and stirred up by its own motion.

But that which is nourished and grows is influenced by a certain regular and equable motion.

And as long as this motion remains in us, so long does sense and life remain; but the moment that it abates and is extinguished, we ourselves decay and perish.
By arguments like these, Cleanthes shows how great is the power of heat in all bodies.

He observes that there is no food so gross as not to be digested in a night and a day; and that even in the excrementitious parts, which nature rejects, there remains a heat.

The veins and arteries seem, by their continual quivering, to resemble the agitation of fire; and it has often been observed when the heart of an animal is just plucked from the body that it palpitates with such visible motion as to resemble the rapidity of fire.