The doctor, however, cannot move a step without the physiologist. His business is to correct the revolt of any organ from its allotted task, and repair the damage done by its deviation from the normal path. This he cannot possibly do if he does not know what that organ’s normal conditions are, and what they are it is the physiologist’s duty to tell him. A doctor, therefore, should be an enlightened physiologist, knowing how the body ought to work, and referring diseases to their real cause, such as the poisons formed by an invasion of bacteria or otherwise, or wrong feeding—that is to say, deficiency or excess of fuel for one of the body’s many engines. Medicine is still to a large extent rule of thumb. We don’t know to what many diseases are due, or why certain things relieve them, if any remedy is known; and until these questions are satisfactorily settled, it is vain to hope that disease as a whole can be successfully combated. It is no use knowing what will stop certain unpleasant symptoms if we do not know how to remove their real cause, and for this end the whole body and every individual component organ is being studied, that the process of life may be accurately understood; and the man who is doing this for his friend the doctor is the _physiologist_. The physiologist has many enemies, a motley array of cranks held together by such noble bonds as general hatred of science and prejudiced ignorance masquerading as scepticism; but he can afford to ignore them, for the very good reason that people cannot get on without him. It is only on account of this that they are mentioned. People say, ‘The doctor is the person who requires a knowledge of physiology; he is the man who is most likely to study it successfully’—presumably by his mistakes—‘and not waste more time on it than is necessary,’ a point about which they are most solicitous. The doctor, however, prefers to trust the physiologist. If he did not, he would have very little time to do anything else. You might as well expect a tailor to make his own cloth before he makes a coat. He will doubtless be able to make better coats if the quality of the cloth supplied him is improved; but if in order to improve the finished article he lays down his scissors and applies his fingers to weaving, his business will be sure to suffer.