No man can be happy unless he is sane, and no man can be sane who searches for what will injure him in place of what is best.
Other Seneca the Younger quotes:
Ignorance is the cause of fear.
No good thing is pleasant to possess, without friends to share it.
Withdraw into yourself, as far as you can. Associate with those who will make a better man of you. Welcome those whom you yourself can improve. The process is mutual; for men learn while they teach.
Fire proves gold, adversity proves men.
The things hardest to bear are sweetest to remember.
There is nothing more despicable than an old man who has no other proof than his age to offer of his having lived long in the world.
Life is long if it is full.
All cruelty springs from weakness.
If you wish to be loved, love.
As is a tale, so is life: not how long it is, but how good it is, is what matters.