Do you ask what is the foundation of a sound mind? It is, not to find joy in useless things. [Ne gaudeas vanis.] I said that it was the foundation; it is really the pinnacle. We have reached the heights if we know what it is that we find joy in and if we have not placed our happiness in the control of externals... Above all, my dear Lucilius, make this your business; learn how to feel joy.
Seneca, Epistle 23