Epictetus, Golden Sayings 107

Until then these sound opinions have taken firm root in you, and you have gained a measure of strength for your security, I counsel you to be cautious in associating with the uninstructed. 

Or else whatever impressions you receive upon the tablets of your mind in the School will day by day melt and disappear, like wax in the sun. Withdraw then somewhere far from the sun, while you have these waxen sentiments.