The Golden Rule, Do unto others as you would that they should do unto you, is the best summary of duty. And the keeping of that rule is possible only in so far as we love others. We must put ourselves in their place, before we can know how to treat them as we would like to be treated. And this putting self in the place of another is the very essence of love. Thou shalt love thy neighbor as thyself includes all social law. Love is the fulfilling of the law. Love takes different forms in different circumstances and in different relations. To the hungry love gives food; to the thirsty drink; to the naked clothes; to the sick nursing; to the ignorant instruction; to the blind guidance; to the erring reproof; to the penitent forgiveness. Indeed, the social virtues which will occupy the remainder of this book are simply applications of love in differing relations and toward different groups and institutions. THE REWARD. +Love the only true bond of union between persons.+--The desire to be in unity with our fellow-men is, as John Stuart Mill tells us, "already a powerful principle in human nature, and happily one of those which tend to become more strong, even without express inculcation, from the influences of advancing civilization. The deeply rooted conception which every individual even now has of himself as a social being, tends to make him feel it one of his natural wants that there should be harmony between his feelings and aims and those of his fellow-creatures." The life of love is in itself a constant realization of this deepest and strongest desire of our nature. Love is the essence of social and spiritual life; and that life of unity with our fellow-men which love creates is in itself love's own reward. "Life is energy of love." Oneness with those we love is the only goal in which love could rest satisfied. For love is "the greatest thing in the world," and any reward other than union with its object would be a loss rather than a gain. THE TEMPTATION. +Kant remarks that a dove, realizing that the resistance of the air is the sole obstacle to its progress, might imagine that if it could only get away from the air altogether, it would fly with infinite rapidity and ease.+--But in fact, if the air were withdrawn for an instant it would fall helpless to the ground. Friction is the only thing the locomotive has to overcome. And if the locomotive could reason it might think how fast it could travel if only friction were removed. But without friction the locomotive could not stir a hair's breadth from the station.