Sextus Empiricus, Against the Professors 36-38
“Next, let’s consider the way we learn, since learning happens wither through experience or through speech. But of these two approaches, experience comes from this which are demonstrable, the demonstrable is clear, and the clear—because it is obvious—is available to all in common. Such perception which is available to all in common is unteachable. Hence, anything apprehended through experience is not teachable.
Speech either corresponds to some meaning or it does not. If it corresponds to no meaning at all, then it teaches nothing. When it does correspond to some meaning it does it either by intrinsic nature or by established convention. It cannot, in truth correspond to meaning by intrinsic nature since not all people understand the same meaning when they hear it (as when the Greeks listen to barbarians or the barbarians listen to Greeks).
If speech signals meaning by convention, it is clear that people who have absorbed before the meanings to which these words correspond will also comprehend them now, and not because they have learned from them something which was not known—it is more like they are resuscitating what they knew before, while those who lack learning of what they don’t know will not do the same.”
τὸ μετὰ τοῦτο ἀπαιτῶμεν τὸν τρόπον τῆς μαθήσεως. ἢ γὰρ ἐναργείᾳ γίνεται ἢ λόγῳ τὰ τῆς διδασκαλίας. ἀλλὰ τούτων ἡ μὲν ἐνάργεια τῶν δεικτῶν ἐστί, τὸ δὲ δεικτὸν φαινόμενον, τὸ δὲ φαινόμενον, ᾗ φαίνεται, κοινῶς πᾶσι ληπτόν, τὸ δὲ κοινῶς πᾶσι ληπτὸν ἀδίδακτον· οὐκ ἄρα τὸ ἐναργείᾳ δεικτὸν διδακτόν. ὁ δὲ λόγος ἤτοι σημαίνει τι ἢ οὐ σημαίνει. καὶ μηδὲν μὲν σημαίνων οὐδὲ διδάσκαλός τινὸς ἐστι, σημαίνων δὲ ἤτοι φύσει σημαίνει τι ἢ θέσει. καὶ φύσει μὲν οὐ σημαίνει διὰ τὸ μὴ πάντας πάντων ἀκούειν, Ἕλληνας βαρβάρων καὶ βαρβάρους Ἑλλήνων ἢ Ἕλληνας Ἑλλήνων ἢ βαρβάρους βαρβάρων· θέσει δὲ εἴπερ σημαίνει, δῆλον ὡς οἱ μὲν προκατειληφότες τὰ καθ᾿ ὧν αἱ λέξεις κεῖνται καὶ ἀντιλήψονται τούτων, οὐ τὸ ἀγνοούμενον ἐξ αὐτῶν διδασκόμενοι, τὸ δ᾿ ὅπερ ᾔδεισαν ἀνανεούμενοι, οἱ δὲ χρῄζοντες τῆς τῶν ἀγνοουμένων μαθήσεως οὐκέτι.