98     Anyone who has a thorough knowledge of, and can appreciate the remarkable difference of, effects on the health of man of every single substance from those of every other, will readily perceive that among them there can be, in a medical point of view, no equivalent remedies whatever, no surrogates. Only those who do not know the pure, positive effects of the different medicines can be so foolish as to try to persuade us that one can serve in the stead of the other, and can in the same disease prove just as serviceable as the other. Thus do ignorant children confound the most essential different things, because they scarcely know their external appearances, far less their real value, their true importance and their very dissimilar inherent properties.