Moral Letters to Lucilius Section 2

Mihi vero idem expedit, quod tibi; aut non sum amicus, nisi quicquid agitur ad te pertinens, meum est. Consortium rerum omnium inter nos facit amicitia. Nec secundi quicquam singulis est nec adversi; in commune vivitur. Nec potest quisquam beate degere, qui se tantum intuetur, qui omnia ad utilitates suas convertit; alteri vivas oportet, si vis tibi vivere.

ou je ne suis pas ton ami, si toute affaire qui te concerne n’est pas la mienne. L’amitié rend tout indivis entre nous : point de succès personnel non plus que de revers : nous vivons sur un fonds commun. Et le bonheur n’est point pour quiconque n’envisage que soi, rapportant tout à son utilité propre : il nous faut vivre pour autrui, si nous voulons vivre pour nous.

But the fact is, the same thing is advantageous to me which is advantageous to you; for I am not your friend unless whatever is at issue concerning you is my concern also. Friendship produces between us a partnership in all our interests. There is no such thing as good or bad fortune for the individual; we live in common. And no one can live happily who has regard to himself alone and transforms everything into a question of his own utility; you must live for your neighbour, if you would live for yourself.