THEAETETUS: Who indeed, Socrates?
SOCRATES: Yes, Theaetetus; and there are plenty of other proofs which will show that motion is the source of what is called being and becoming, and inactivity of not-being and destruction; for fire and warmth, which are supposed to be the parent and guardian of all other things, are born of movement and of friction, which is a kind of motion;—is not this the origin of fire?
THEAETETUS: It is.
SOCRATES: And the race of animals is generated in the same way?
THEAETETUS: Certainly.
SOCRATES: And is not the bodily habit spoiled by rest and idleness, but preserved for a long time by motion and exercise?
THEAETETUS: True.
SOCRATES: And what of the mental habit? Is not the soul informed, and improved, and preserved by study and attention, which are motions; but when at rest, which in the soul only means want of attention and study, is uninformed, and speedily forgets whatever she has learned?
THEAETETUS: True.
SOCRATES: Then motion is a good, and rest an evil, to the soul as well as to the body?