He who knows enough of things to value them at their true worth never says too much; for he can also judge of the attention bestowed on him and the interest aroused by what he says.
 Jean-Jacques Rousseau, Emile, or On Education (1762). copy citation

add
Author Jean-Jacques Rousseau
Source Emile, or On Education
Topic value interest
Date 1762
Language English
Reference
Note Translated by Barbara Foxley
Weblink http://www.gutenberg.org/cache/epub/5427/pg5427-images.html

Context

“A great flow of words comes either from a pretentious spirit, of which I shall speak presently, or from the value laid upon trifles which we foolishly think to be as important in the eyes of others as in our own. He who knows enough of things to value them at their true worth never says too much; for he can also judge of the attention bestowed on him and the interest aroused by what he says. People who know little are usually great talkers, while men who know much say little. It is plain that an ignorant person thinks everything he does know important, and he tells it to everybody. But a well-educated man is not so ready to display his learning; he would have too much to say, and he sees that there is much more to be said, so he holds his peace.
source