As the action of drugs upon animals is to a certain extent different from that on man, it is undoubtedly desirable to ascertain the action of drugs by experiments upon healthy man. This is all the more necessary because by experiments upon animals we are able to discover only the ruder differences between drugs, and we cannot ascertain the finer shades of action, both because it is in man alone that these finer differences occur, and because it is he alone who can give information regarding slight changes which he can perceive in his own organism, but which are imperceptible to others who may be observing him. There is no doubt that many observers of this sort, several of whom have been homoeopathists, have done good service to medicine by carefully noting and carefully comparing the symptoms produced by various drugs. These observations, however, are liable to fallacies, as I will presently mention.