The first principle which runs through all art and nature is simplicity; this also is to be the rule of human life.
The second stage of education is gymnastic, which answers to the period of muscular growth and development.

The simplicity which is enforced in music is extended to gymnastic; Plato is aware that the training of the body may be inconsistent with the training of the mind, and that bodily exercise may be easily overdone.

Excessive training of the body is apt to give men a headache or to render them sleepy at a lecture on philosophy, and this they attribute not to the true cause, but to the nature of the subject.

Two points are noticeable in Plato's treatment of gymnastic:--First, that the time of training is entirely separated from the time of literary education.

He seems to have thought that two things of an opposite and different nature could not be learnt at the same time.