Relationships of friendship and love are not necessarily of a symmetrical pattern. Friendship is something which is impossible to define exactly. Friendship, may often exist between persons of different status, between father and son, or between educated and uneducated people. Aristotle goes on to say that friendship is not a casual thing: friendship takes time: in the Greek manner he adds that there is nothing so characteristic of friendship as living together. Everything in personal relations which has depth and significance, can be incorporated and transmuted and added to, in the special relation of affinity between a man and a woman who love each other. The psycho-physical intercourse of love is a giving and a receiving-a mutual surrender, and mutual acceptance, by each of the other. Professor Baillie in Our Knowledge of God discusses the relation between the I-Thou conception as applied to human affairs, and its bearing upon our knowledge of God.