*6* Although these truths are so self-evident that the subject does not call for discussion, still I have discussed it in another connection. If, therefore, these schools should claim to be consistent, they could not say anything about duty; and no fixed, invariable, natural rules of duty can be posited except by those who say that moral goodness is worth seeking solely or chiefly for its own sake. Accordingly, the teaching of ethics is the peculiar right of the Stoics, the Academicians, and the Peripatetics; for the theories of Aristo, Pyrrho, and Erillus have been long since rejected; and yet they would have the right to discuss duty if they had left us any power of choosing between things, so that there might be a way of finding out what duty is. I shall, therefore, at this time and in this investigation follow chiefly the Stoics, not as a translator, but, as is my custom, I shall at my own option and discretion draw from those sources in such measure and in such manner as shall suit my purpose. *7* Since, therefore, the whole discussion is to be on the subject of duty, I should like at the outset to define what duty is, as, to my surprise, Panaetius has failed to do. For every systematic development of any subject ought to begin with a definition, so that every one may understand what the discussion is about. III. Omnis de officio duplex est quaestio: unum genus est, quod pertinet ad finem bonorum, alterum, quod positum est in praeceptis, quibus in omnis partis usus vitae conformari[9] possit. Superioris generis huius modi sunt exempla: omniane officia perfecta sint, num quod officium aliud alio maius sit, et quae sunt generis eiusdem. Quorum autem officiorum praecepta traduntur, ea quamquam pertinent ad finem bonorum, tamen minus id apparet, quia magis ad institutionem vitae communis spectare videntur; de quibus est nobis his libris explicandum. Atque etiam alia divisio est officii. *8* Nam et medium quoddam officium dicitur et perfectum. Perfectum officium rectum, opinor, vocemus, quoniam Graeci κατόρθωμα, hoc autem commune officium καθῆκον vocant.[10] Atque ea sic definiunt, ut, rectum quod sit, id officium perfectum esse definiant; medium autem officium id esse dicunt, quod cur factum sit, ratio probabilis reddi possit.