Lastly, since mental phenomena are =correlated with the function of the nervous system=, the psychologist’s task is not complete until he has acquainted himself with the physiology of that system, and has worked out the correlation as accurately as is possible. Here, again, is something that you will better understand when you have read further in the book. For the present we will notice two points. First, the psychologist can gain access to a large part of his world only by way of the organs of sense; and it is therefore important that he know the structure and functions of these organs and their relation to the brain. Secondly, a train of mental phenomena may be guided and directed by events, occurring within the nervous system, which themselves have no counterpart in the world of mind; for, while all mental phenomena are correlated with processes in the nervous system, not all processes in the nervous system have mental phenomena to correspond with them. Unless, then, the psychologist knows the nature of these guiding events, he will be like the chemist who failed to take account of temperature; he will lack knowledge of relevant circumstances. Special books upon the nervous system have been written, giving in outline what the student of psychology needs to know; some of them are referred to at the end of the chapter; but it is an advantage to have taken a practical course in the physiology of the nervous system, and to be able to think in terms of neural processes. If you have had no such opportunity you can still learn a good deal from diagrams and verbal accounts; and you may find comfort in the assurance that there have been eminent psychologists who knew very little about the brain. In fine, then, the problem of human psychology is threefold: to analyse mental phenomena into their elements, to discover the laws of mental connection, and to work out in detail and under all its phases the correlation of mind with nervous system. § 5. =The Method of Psychology.=—Having learned what we have to do, let us ask what method we are to follow in doing it. So far as the nervous system is concerned, it is evident that the psychologist must take his cue from the physiologist; indeed, this part of his problem makes him, for the time being, a physiologist, only that his real interest remains centred in mind. But how is it when he is attacking the other parts of the problem? Is there a special psychological method, a peculiar way of working, that he must adopt in his study of mental phenomena? The answer is No: his =method is that of science in general=.