We ought to esteem it of the greatest importance that the fictions which children first hear should be adapted in the most perfect manner to the promotion of virtue.
It's like this, I think: the excellence of a good body doesn't make the soul good, but the other way around: the excellence of a good soul makes the body as good as it can be.
What the expression is intended to mean, I think, is that there is a better and a worse element in the character of each individual, and that when the naturally better element controls the worse then the man is said to be "master of himself", as a term of praise. But when - as a result of bad upbringing or bad company one s better element is overpowered by the numerical superiority of one s worse impulses, then one is criticized for not being master of oneself and for lack of self control.
The true lover of knowledge naturally strives for truth, and is not content with common opinion, but soars with undimmed and unwearied passion till he grasps the essential nature of things.
If anyone comes to the gates of poetry and expects to become an adequate poet by acquiring expert knowledge of the subject without the Muses' madness, he will fail, and his self-controlled verses will be eclipsed by the poetry of men who have been driven out of their minds.