33.  For our treatment does not correspond with virtue and vice, one of which is most excellent and beneficial at all times and in all cases, and the other most evil and harmful; and, instead of one and the same of our medicines invariably proving either most wholesome or most dangerous in the same cases—be it severity or gentleness, or any of the others which we have enumerated—in some cases it proves good and useful, in others again it has the contrary effect, according, I suppose, as time and circumstance and the disposition of the patient admit.  Now to set before you the distinction between all these things, and give you a perfectly exact view of them, so that you may in brief comprehend the medical art, is quite impossible, even for one in the highest degree qualified by care and skill:  but actual experience and practice are requisite to form Are requisite to form, lit., by ‘actual…they become clear to.’ a medical system and a medical man.