Sextus Empiricus Pyrrhoniae Hypotyposes 3.22.174

[174] For just as the man who has no notion of ‟horse” has no knowledge of what ‟neighing” is and cannot arrive thereby at a notion of ‟horse,” unless he should first meet with a neighing horse, so too one who is seeking the essence of the good, because he has no knowledge of the good, cannot perceive the attribute which is peculiar to it alone in order that he may be enabled thereby to gain a notion of the good itself. For he must first learn the nature of the good itself, and then pass on to apprehend that it is useful, and that it is choiceworthy for its own sake, and that it is productive of happiness.