The practical application of psychology has certain well-defined spheres. Its bearing upon education has long been recognised, and much valuable work done in relation to the study of the child mind. The psychology of fatigue, in relation to industrial efficiency, has also found recognition as a branch of applied science not without its practical value. The field of social problems is still largely awaiting exploration, and there can be little doubt that the study of the psychology of the criminal and unemployable would yield results of the greatest social value. At the present moment, it is the field of abnormal psychology that holds the focus of attention. That inestimably valuable results are being obtained in this field of study no one can dispute, but its value is not confined to the relief of disease alone, but, as the research is progressing deeper, to the revelation of the conditions that give rise to disease. Just as the study of pathology gave us the science of hygiene, so the study of mental diseases is showing us the way to healthier thinking. It is teaching us that any abnormal attitude towards life will produce mental discomfort, if not actual disease, and it is showing us, just as physiological hygiene has shown us, that if the developing intelligence of man leads him to depart from primitive conditions wherein the instincts are sufficient guides, then he must also apply his reason to the new problems to which the new conditions give rise, and not leave the solution of these to instincts which are only fitted for the simplest form of functioning. The instinct of combativeness, or the instinct of flight, will not conduct the evolutions of a modern army, and neither will the primitive impulses enable man to live well and happily in conditions which elaborate mental processes have built up--as witness the terrible prevalence of unsolved sex problems beneath the fair show of our civilisation. Two-thirds, if not more, of nerve trouble have their origin in the efforts of a primitive instinct to function under civilised conditions and its failure to make the adaptation. We need to take our instincts out of the region of the subconscious and apply our reason to them if we are to solve the problems that press upon us. Throughout this book it will have been seen that stress has been laid upon the functioning and activity of those levels of the mind that are below the threshold of consciousness, and that it has been pointed out that the instincts, and not the reason, are the key to the human mind. But it has also been shown that the mind is in a state of evolution, and that reason, as its latest development, has an equal biological significance with the instincts of sex and self-preservation, and that we can no more afford to ignore the higher attributes of the human mind than we can afford to deny their true place to the primitive.