Letters 66.14

“What then,” you say; “is there no difference between joy and unyielding endurance of pain?” None at all, as regards the virtues themselves; very great, however, in the circumstances in which either of these two virtues is displayed. In the one case, there is a natural relaxation and loosening of the soul; in the other there is an unnatural pain. Hence these circumstances, between which a great distinction can be drawn, belong to the category of indifferent things, but the virtue shown in each case is equal.

Seneca, Letters to Lucilius 66.14, translated by Richard Mott Gummere