Aurea mediocritas. Seneca, Epistles 4.39.4-5

The mind is always moving, so we must direct its movement toward better things. These will not be excessive things: too much success is bad, harming the interests that smaller success would help. We want to keep our wins from getting too big. To avoid being seduced into pursuing more goods than would be truly good for us.

Quemadmodum flamma surgit in rectum, iacere ac deprimi non potest, non magis quam quiescere, ita noster animus in motu est, eo mobilior et actuosior quo vehementior fuerit. Sed felix qui ad meliora hunc impetum dedit: ponet se extra ius dicionemque fortunae; secunda temperabit, adversa comminuet et aliis admiranda despiciet. Magni animi est magna contemnere ac mediocria malle quam nimia; illa enim utilia vitaliaque sunt, at haec eo quod superfluunt nocent. Sic segetem nimia sternit ubertas, sic rami onere franguntur, sic ad maturitatem non pervenit nimia fecunditas. Idem animis quoque evenit quos immoderata felicitas rumpit, qua non tantum in aliorum iniuriam sed etiam in suam utuntur.

Even as a flame rises ever erect, incapable of lying flat or being pressed down as it refuses to go out, so our mind is always in motion, becoming more mobile and active as it grows stronger. Happy the man who turns its impulse to better things, placing himself thus beyond the rule and power of fortune. He will tame his success, break whatever opposes him, and despise what others are compelled to admire. It is the mark of a great mind to despise great things, and to prefer the middling or mediocre to anything excessive. Mediocre things are really useful and vital, but excess causes harm. Too much fertilizer destroys your crop. Branches overladen with fruit are broken. What is overly fecund in conceiving never brings its offspring to maturity. In the same way, an extravagant happiness breaks the minds whose overweening harms not merely others, but also themselves.