A more or less accurately defined, abstract expression for so fundamental a principle of all knowledge must have been found at a very early age; it would, therefore, be difficult, and besides of no great interest, to determine where it first appeared. Neither Plato nor Aristotle have formally stated it as a leading fundamental principle, although both often speak of it as a self-evident truth. Thus, with a _naïveté_ which savours of the state of innocence as opposed to that of the knowledge of good and of evil, when compared with the critical researches of our own times, Plato says: ἀναγκαῖον, πάντα τὰ γιγνόμενα διά τινα αἰτίαν γίγνεσθαι· πῶς γὰρ ἂν χωρὶς τούτων γίγνοιτο;[16] (_necesse est, quæcunque fiunt, per aliquam causam fieri: quomodo enim absque ea fierent?_) and then again: πᾶν δὲ τὸ γιγνόμενον ὑπ' αἰτίου τινὸς ἐξ ἀνάγκης γίγνεσθαι· παντὶ γὰρ ἀδύνατον χωρὶς αἰτίου γένεσιν σχεῖν[17] (_quidquid gignitur, ex aliqua causa necessario gignitur: sine causa enim oriri quidquam, impossibile est_). At the end of his book "De fato," Plutarch cites the following among the chief propositions of the Stoics: μάλιστα μὲν καὶ πρῶτον εἶναι δόξειε, τὸ μηδὲν ἀναιτίως γίγνεσθαι, ἀλλὰ κατὰ προηγουμένας αἰτίας[18] (_maxime id primum esse videbitur, nihil fieri sine causa, sed omnia causis antegressis_). [16] Platon, "Phileb." p. 240, ed Bip. "It is necessary that all which arises, should arise by some cause; for how could it arise otherwise?" [Tr.'s add.] [17] _Ibid._ "Timæus," p. 302. "All that arises, arises necessarily from some cause; for it is impossible for anything to come into being without cause." [Tr.'s add.] [18] "This especially would seem to be the first principle: that nothing arises without cause, but [everything] according to preceding causes." [Tr.'s add.]