Sextus Empiricus Pyrrhoniae Hypotyposes 2.18.214

[214] Now they at once assert that the sciences of natural objects exist whereas those of conventional objects have no existence, and that with reason. For science claims to be a thing that is firm and invariable, but the conventional objects are easily liable to change and variation, because their character is altered by the shifting of the conventions which depend upon ourselves. Since, then, the significance of names is based on convention and not on nature ¹ (for otherwise all men, barbarians as well as Greeks, would understand all the things signified by the terms, besides the fact that it is in our power at any time to point out and signify the objects by any other names we may choose), how would it be possible for a science capable of dividing a name into its significations to exist? Or how could Dialectic really be, as some imagine, a ‟science of things which signify and are signified?”