Theologians maintain that just the presence of a feeling of God in Man's heart, is, in itself, a proof of His existence. Since -as they assure us- that feeling is innate, it's actually a reminiscence. Well, if it's as they say, Blanca, then, along with a feeling of God (and, as I hope to demonstrate during the course of these letters, closely bound to it), there exists in Man's heart another innate feeling of no less power. The feeling of the twin soul, of the one creature who, out of every other, is destined to us, for it's the other half that will us complete us." Thus begins one of the letters of this epistolary essay in which the author undertakes an exhaustive tracing of the theory of soul mates in the history of Religion and Philosophy, of Literature and Occult Sciences, showing the preeminent place that in the worldview of the ancient sages occupied this enigmatic feeling that we know today with the name of romantic love.