I. ALL who, on attempting to speak or to write on medicine, have assumed for themselves a postulate as a basis for their discussion$-$heat, cold, moisture, dryness, or anything else that they may fancywho narrow down the causal principle of diseases and of death among men, and make it the same in all cases, postulating one thing or two, all these obviously blunder in many points even of their statements, ${?}^1$ but they are most open to censure because they blunder in what is an art, and one which all men use on the most important occasions, and give the greatest honours to the good craftsmen and practitioners in it$.$ Some practitioners are poor, others very excellent; this would not be the case if an art of medicine did not exist at all, and had not been the subject of any research and discovery, but all would be equally inexperienced and unlearned therein, and the treatment of the sick would be in all respects haphazard. But it is not so; just as in all other arts the workers vary much in skill and in knowledge, ${?}^2$ so also is it in the case of medicine. Wherefore I have deemed that it has