The relation of a brother and a sister, especially if they do not marry, appears to me of a very singular nature. It is a familiar and tender friendship with a female, much about our own age; an affection perhaps softened by the secret influence of sex, but pure from any mixture of sensual desire, the sole species of platonic love that can be indulged with truth, and without danger.

Edward Gibbon, Memoirs of My Life and Writings, London, 1796