for (A) if good condition is known, bad condition also becomes known, and (B) good condition is known from the things that are in good condition, and they from it.
(15) adibidez, osasunetik abiatuz ez da kontrakoa egiten, osasungarria dena baizik, besterik ez; izan ere, era osasungarrian ibiltzen garela esaten dugu osasun onekoa den baten moduan ibiltzen garenean.