The laws of our human constitution compel us to recognise that intellect and conscience, although essential parts, are not identical parts of our nature. Long experience shows us that social progress can only become permanent when conscience guides intelligence. How far the guidance of conscience can extend, with the practical results to medical research involved in the recognition of such guidance, forms the subject of present consideration. CHAPTER I _The Growth of Conscience_ It is through the gradual and harmonious development of intelligence with that element in our nature that we name conscience that the human race passes from lower to higher states of civilization. In pursuing our ideals, conscience is our instinctive monitor of right and wrong. Our great naturalist, Darwin, laid down as a law of evolution that ‘the moral sense, or conscience, is by far the most important of the differences between man and the lower animals. Duty--“ought”--is the most noble of all the attributes of man.’ Victor Hugo, with the prophetic insight of genius, calls conscience ‘that modicum of innate science with which each one is born.’ The growth of human conscience in its perception of justice and in its sympathetic relation to creation is the surest measure of individual and national progress. Various intellectual theories may be formed as to the origin and growth of conscience. It may be held to be intuitive, springing up as inevitably as the instinctive feelings born with the natural relations of life; or it may be looked upon as gradually evolved, the ‘result of countless experiences of fear, love, utility, transmitted through generations.’ But however originating, conscience is a positive and potent fact. It is, indeed, the mightiest factor in social life. It is the great controller of selfhood. It enlarges human character and guides human conduct. The deepening of this principle through the growth of justice and sympathy marks an advancement in the type of humanity. Increasing respect for life is one of the clearest signs of growing conscience. Our reverence for the principle of life grows with our enlarging intellectual perception of its universality and its unlimited power of development. As life is marked by activity, and cannot remain stationary, so conscience shares this law of life. It must inevitably advance or retrograde.