Aristotle: De Interpretatione (On Interpretation)

Τὸ δὲ μὴ ὄν, ὅτι δοξαστόν, οὐκ ἀληθὲς εἰπεῖν ὄν τι· δόξα γὰρ αὐτοῦ οὐκ ἔστιν ὅτι ἔστιν, ἀλλ᾿ ὅτι οὐκ ἔστιν.

Τούτων δὲ διωρισμένων σκεπτέον ὅπως ἔχουσιν αἱ ἀποφάσεις καὶ καταφάσεις πρὸς ἀλλήλας αἱ τοῦ δυνατὸν εἶναι καὶ μὴ δυνατὸν καὶ ἐνδεχόμενον καὶ μὴ ἐνδεχόμενον, καὶ περὶ τοῦ ἀδυνάτου τε καὶ ἀναγκαίου· ἔχει γὰρ ἀπορίας τινάς. Εἰ γὰρ τῶν συμπλεκομένων αὗται ἀλλήλαις ἀντίκεινται ἀντιφάσεις, ὅσαι κατὰ τὸ εἶναι καὶ μὴ εἶναι τάττονται, οἷον τοῦ εἶναι ἄνθρωπον ἀπόφασις τὸ μὴ εἶναι ἄνθρωπον, οὐ τὸ εἶναι μὴ ἄνθρωπον, καὶ τοῦ εἶναι λευκὸν ἄνθρωπον τὸ μὴ εἶναι λευκὸν ἄνθρωπον, ἀλλ᾿ οὐ τὸ εἶναι μὴ λευκὸν ἄνθρωπον. Εἰ γὰρ κατὰ παντὸς ἡ κατάφασις ἢ ἡ ἀπόφασις, τὸ ξύλον ἔσται ἀληθὲς εἰπεῖν εἶναι μὴ λευκὸν ἄνθρωπον. Εἰ δὲ τοῦτο οὕτως, καὶ ὅσοις τὸ εἶναι μὴ προστίθεται, τὸ αὐτὸ ποιήσει τὸ ἀντὶ τοῦ εἶναι λεγόμενον, οἷον τοῦ ἄνθρωπος βαδίζει οὐ τὸ οὐκ ἄνθρωπος βαδίζει ἀπόφασις ἔσται, ἀλλὰ τὸ οὐ βαδίζει ἄνθρωπος· οὐδὲν γὰρ διαφέρει εἰπεῖν ἄνθρωπον βαδίζειν ἢ ἄνθρωπον βαδίζοντα εἶναι. Ὥστε εἰ οὕτως πανταχοῦ, καὶ τοῦ δυνατὸν εἶναι ἀπόφασις ἔσται τὸ δυνατὸν μὴ εἶναι, ἀλλ᾿ οὐ τὸ μὴ δυνατὸν εἶναι. Δοκεῖ δὲ τὸ αὐτὸ δύνασθαι καὶ εἶναι καὶ μὴ εἶναι· πᾶν γὰρ τὸ δυνατὸν τέμνεσθαι ἢ βαδίζειν καὶ μὴ βαδίζειν καὶ μὴ τέμνεσθαι δυνατόν. Λόγος δέ, ὅτι ἅπαν τὸ οὕτω δυνατὸν οὐκ ἀεὶ ἐνεργεῖ, ὥστε ὑπάρξει αὐτῷ καὶ ἡ ἀπόφασις· δύναται γὰρ καὶ μὴ βαδίζειν τὸ βαδιστικὸν καὶ μὴ ὁρᾶσθαι τὸ ὁρατόν. Ἀλλὰ μὴν ἀδύνατον κατὰ τοῦ αὐτοῦ ἀληθεύεσθαι τὰς ἀντικειμένας φάσεις· οὐκ ἄρα τοῦ δυνατὸν εἶναι ἀπόφασίς ἐστι τὸ δυνατὸν μὴ εἶναι. Συμβαίνει γὰρ ἐκ τούτων ἢ τὸ αὐτὸ φάναι καὶ ἀποφάναι ἅμα καὶ κατὰ τοῦ αὐτοῦ, ἢ μὴ κατὰ τὸ εἶναι καὶ μὴ εἶναι τὰ προστιθέμενα γίνεσθαι φάσεις καὶ ἀποφάσεις. Εἰ οὖν ἐκεῖνο ἀδύνατον, τοῦτ᾿ ἂν εἴη αἱρετόν. Ἔστιν ἄρα ἀπόφασις τοῦ δυνατὸν εἶναι τὸ μὴ δυνατὸν εἶναι. Ὁ δ᾿ αὐτὸς λόγος καὶ περὶ τοῦ ἐνδεχόμενον