The connection between theoretical and practical ethics is very close and vital. Theory and practice always affect each other. They cannot be held wholly apart. They act and react on each other, in ceaseless influences. It is so in every department of thought and life. Theoretical error in physical science, in art, in trade, in political economy, can hardly fail to appear in faulty or misdirected practice. Every failure to grasp first principles correctly and firmly is sure to mean failure also to grasp and maintain the true order and beauty of right living. False views as to the reality and grounds of moral obligation weaken, vitiate and corrupt life. They become, often, the fountain of far-flowing streams of evil and blight. At best they lack power for the true and right life. A correct theory of ethical truth is, therefore, demanded by all the high interests in the moral life of man and the order of the world. It is apparent, however, that it is only the theoretical part that constitutes the science of ethics. It alone settles the systematic view of it, revealing its underlying pre-suppositions and principles, exhibiting its reasons and determining its laws. Practical ethics, as simply pointing out how these bear upon men's temper and conduct in actual life, is apart from the scientific investigation. Though this has usually constituted a large portion of formal treatises on the subject, we, for the reason thus given, omit it from this discussion. =4.= A few of the relations of ethical science are properly called to mind here. Its place will thus be more clearly seen. It sustains very close relations to three other great branches of study. ==Relation to Psychology.== (1) To psychology. It is organically related to this. In that part of its work which determines the reality and nature of moral obligation from the constitution and action of the human mind, moral science joins with psychology in the investigation of man's mental capacities and powers. Ever since the days of Aristotle ethics has been seen to have real psychological basis and pre-suppositions.[5] Yet, as a result of the later general subsuming of ethics under religious and theological precepts, this basis received but little distinct investigation till the time of Shaftesbury who, though failing to give adequate or correct account of it, brought it prominently forward. Mental science is essentially conditional for moral science. The behests of duty are provided for and sanctioned in human nature. The moral discriminations and convictions emerge as psychical phenomena. "To understand what man ought to do, it is necessary to know what he is." In the very structure and adjustment of his powers, it becomes apparent that he has been made for duty and organized under obligations.