 If we regard Alcmaeon as primarily a doctor or medical thinker, rather than as a cosmologist, “things that are not perceptible concerning mortals” is likely to refer to the interior of the body and hidden diseases. Passages in the Greek medical writings of the fifth century provide clear parallels for the difficulty of knowing about the interior of the body and invisible maladies (Gemelli Marciano 2007, 20–21). On this reading Fr.1 is addressed to medical students. Parallels with medical treatises suggest that, after first raising difficulties about medical knowledge in these matters, Alcmaeon may have gone on to assert that these difficulties can be overcome with the proper teaching, the teaching that followed in his book. If this is the correct context in which to read the fragment, it is not so much about the limits of understanding as the success of medical teaching in overcoming apparent limits.