Letter 20: Aeneas combines praise of friendship with a diagnosis for Gessius.

Excellent Nemesion is praised for his knowledge of the laws, but he is admired even more for his character. What is the clearest sign of this? He knows how to hunt for friends: he rejoices with them when things go well and grieves with them when things happen as they should not. I would like you to practice this too, alongside your other philosophy. You will practice it if you remember absent friends as well as present ones. As long as diseases are hidden, the remedies lying in the open are hidden too, and error rules the treatment. But knowledge of the disease shines like a light on the art, opens the abundance of remedies, and makes the treatment no longer uncertain. I described the illness, if you remember: frequent motion came first, then a great flow of blood in place of excess fluid, then rest and diet covered the disease over; then motion again, blood again, rest and diet again, until the illness could no longer hide and constriction pressed in, moderate at first, then not slight, and finally unbearable, striking within and tearing around the navel. Why should anyone conceal from you what nature is not ashamed to publish? One remedy remains to fight the illness alone, unless you discover another by your wisdom: the woven circle of an herb that the physicians' boys solemnly call physallis. This alone may be able to fend off the disease's ambushes and traps.