In the later novel, The Cat Who Walks Through Walls, a character refers to the earlier book’s comment: “Dr. Harshaw says that ‘the word ‘love’ designates a subjective condition in which the welfare and happiness of another person are essential to one’s own happiness.”

Love is the condition in which the happiness of another person is essential to your own.

— Robert Anson Heinlein, Stranger in a Strange Land (1961)