What is philosophy? Seneca, Epistles 2.16.2-3
What is philosophy? Seneca makes it sound like μῆτις or σωφρσύνη, two different ways of conceiving practical wisdom. One thing is certain: it requires action. You will not find it in sterile wordplay.
Itaque tibi apud me pluribus verbis aut adfirmatis nec tam longis (†) nil opus; intellego multum te profecisse. Quae scribis unde veniant scio; non sunt ficta nec colorata. Dicam tamen quid sentiam: iam de te spem habeo, nondum fiduciam. Tu quoque idem facias volo: non est quod tibi cito et facile credas. Excute te et varie scrutare et observa; illud ante omnia vide, utrum in philosophia an in ipsa vita profeceris.
Non est philosophia populare artificium nec ostentationi paratum; non in verbis sed in rebus est. Nec in hoc adhibetur, ut cum aliqua oblectatione consumatur dies, ut dematur otio nausia: animum format et fabricat, vitam disponit, actiones regit, agenda et omittenda demonstrat, sedet ad gubernaculum et per ancipitia fluctuantium derigit cursum. Sine hac nemo intrepide potest vivere, nemo secure; innumerabilia accidunt singulis horis quae consilium exigant, quod ab hac petendum est.
Thus there is no need for you to respond to me with more words of encouragement, or even as many as you sent; I understand that you made great progress. I know what you write, and whence it comes. Your words are not forced or false. I will tell you what I feel: I have hope in you already, though not complete confidence yet. I desire you to do exactly as I have done: it is not right to trust yourself quickly, and easily. Shake yourself thoroughly. Consider yourself from all sides, and watch. Above all else, look to see whether your progress lies in philosophy, or in life itself.
Philosophy is no common profession, nor is it something we undertake for ostentation. It is not in words, but in things. It is not something we employ to spend our days pleasantly, or to replace our illness with leisure. It shapes the mind and forges it. It orders our life and rules our actions. It shows us what must be done, and what avoided. It sits at the helm, and directs our course through the treacherous hazards of life's rolling sea. Without it, no man can live boldly, or secure. Countless perils befall us every hour which demand counsel, which we must seek from philosophy.
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(†) Madvig offers a plausible emendation: verbis aut adfirmatione iam.