Epictetus, Golden Sayings 108

We must approach this matter in a different way; it is great and mystical. It is no common thing, nor given to every man. 

Wisdom alone, it may be, will not suffice for the care of youth: a man needs also a certain measure of readiness—an aptitude for the office, yes, and certain bodily qualities, and above all, to be counseled of God Himself to undertake this post.

Even as He counseled Socrates to fill the post of one who confutes error, assigning to Diogenes the royal office of high reproof, and to Zeno that of positive instruction. 

Whereas you would be pleased to set up for a physician provided with nothing but drugs! Where and how they should be applied, you neither know nor care.