Who is happy? Seneca, Epistles 5.45.9

Seneca describes the truly happy man for Lucilius. He is a diligent student of nature, not the darling of fortune. He does not care if the bank is solvent or not.

Si utique vis verborum ambiguitates diducere, hoc nos doce, beatum non eum esse quem vulgus appellat, ad quem pecunia magna confluxit, sed illum cui bonum omne in animo est, erectum et excelsum et mirabilia calcantem, qui neminem videt cum quo se commutatum velit, qui hominem ea sola parte aestimat qua homo est, qui natura magistra utitur, ad illius leges componitur, sic vivit quomodo illa praescripsit; cui bona sua nulla vis excutit, qui mala in bonum vertit, certus iudicii, inconcussus, intrepidus; quem aliqua vis movet, nulla perturbat; quem fortuna, cum quod habuit telum nocentissimum vi maxima intorsit, pungit, non vulnerat, et hoc raro; nam cetera eius tela, quibus genus humanum debellatur, grandinis more dissultant, quae incussa tectis sine ullo habitatoris incommodo crepitat ac solvitur.

If you really want to dissolve the ambiguity of words, show us how the man the mob calls happy is no such thing, despite the huge funds flowing into his pockets. Teach us to recognize the truly happy man: one whose happiness exists wholly in his mind, who stands erect and exalted as he tramples the mob's miraculous money underfoot. He judges a man only by the part of us that is actually human, making nature his teacher. Composing himself in accordance with her laws, he lives as she has ordained. No force ever drives his goods to evil. He turns every evil to good account, meeting it head-on, certain of his judgement & unshakeable in his conviction. Even if some blow manages to move him, it never breaks his calm. Though fortune pierce him through with her most vicious dart, still she cannot wound him, and it is rare that she finds him open to her onslaught. The majority of her projectiles, by which the human race is utterly beaten down, glance off him like hail striking well-made roofs, thundering and falling without causing any trouble to folk safely inside.