Epictetus, Golden Sayings 69

I think I know now what I never knew before—the meaning of the common saying, "A fool you can neither bend nor break."

Pray heaven I may never have a wise fool for my friend! There is nothing more intractable.

"My resolve is fixed!"

Why, so madmen say too; but the more firmly they believe in their delusions, the more they stand in need of treatment.