A man, for instance, visits and relieves one who is sick. The action, which is so interesting and important, may be ascribed either to the principle of love, or the sentiment of duty. The father of a family restrains those under his care from outward labors on the Sabbath day, and visits the house of God with them; and, in doing so, he may be moved by love to God, or merely by the constraint of mental conviction and obligation. A child may render obedience to his parents from either of these motives; either because he loves to obey, — it being a pleasure, a delight to him to obey,— or because, without love, and sometimes against love, he feels it to be his duty to obey. And thus of many other instances.
It is important to ascertain the true position and the comparative relations of these principles. In the order of nature, love is the first in time. The heart naturally operates before the conscience. One evidence of this is, that it is the office of the conscience to intimate the proper regulations, and to establish the law of the heart. It is obvious, however, that there can be no regulation without something which is regulated; and conscience, whose business it is to regulate and direct, would obviously be a faculty without application and without use, if there were not propensities and affections which in the order of nature operated antecedently. Love is the true impulsive principle, the central movement or life of man, as it is of God and of all holy beings. Of conscience, it can only be said that it is its guard, the flaming sword which waves and flashes round it to protect its purity. And he who does not act in the right way naturally, and by the power of his own loving life, must be wounded and goaded into the right by the authority and the penalties of the moral sense.
— A Treatise on Divine Union (1851) Part 4, Chapter 8.