... we must take care, my friend, that the Sophist does not deceive us
when he praises what he sells, like the dealers wholesale or retail who
sell the food of the body; for they praise indiscriminately all their
goods, without knowing what are really beneficial or hurtful:  neither
do their customers know, with the exception of any trainer or physician
who may happen to buy of them.  In like manner those who carry about the
wares of knowledge, and make the round of the cities, and sell or retail
them to any customer who is in want of them, praise them all alike; though
I should not wonder, O my friend, if many of them were really ignorant
of their effect upon the soul; and their customers equally ignorant,
unless he who buys of them happens to be a physician of the soul.  If,
therefore, you have understanding of what is good and evil, you may safely
buy knowledge of Protagoras or of any one; but if not, then, O my friend,
pause, and do not hazard your dearest interests at a game of chance.
For there is far greater peril in buying knowledge than in buying meat
and drink:  the one you purchase of the wholesale or retail dealer, and
carry them away in other vessels, and before you receive them into the
body as food, you may deposit them at home and call in any experienced
friend who knows what is good to be eaten or drunken, and what not,
and how much, and when; and then the danger of purchasing them is not
so great.  But you cannot buy the wares of knowledge and carry them away
in another vessel; when you have paid for them you must receive them into
the soul and go your way, either greatly harmed or greatly benefited;
and therefore we should deliberate and take counsel with our elders;
for we are still young--too young to determine such a matter.  

Plato, Protagoras, 313c-314b, trans. by Jowett