Apart from passing references in the Dialogue on the Two Major Systems of the World, Galileo did not leave us a systematic examination of perpetual motion, probably because he recognized its impossibility. In his view, only a circular, uniform motion such as that of the planets could be perpetual—and surely not a rectilinear motion, constantly accelerated or slowed. To be in perpetual motion, the object itself would need to be incorruptible and eternal. Both conditions are necessary, but are never concomitant in nature. Galileo’s replies to a few letters describing perpetual-motion machines are lost, but we may assume that he urged his correspondents to search for the real causes of the motion produced by the machines, as he believed no human artifice could violate nature’s laws.