Liber, qui sui causa est. That causa is meant by St Thomas for the nominative case, is clear from the context here, as also from B. I, Chap. LXXXVIII (ad fin in the Latin); also B. II, Chap. XLVIII, n. 2, where the same definition is quoted. But turning to the original, Aristotle, Metaph. I, ii, eleutheros anthropos ho hautou heneka kai mê allou ôn (he is a free man, who is for his own sake, and not for the sake of another), we find sui causâ, hautou heneka, 'for his own sake.'
Of God and His Creatures: 4.22