On a spring afternoon in the year two thousand, I happened to wander into a bookshop in the old Barri Gòtic in Barcelona. The owner was busy taking books from two large wooden boxes. I was curious, so I asked him if I could have a look. The books were in Catalan, Spanish, French, and English, some of them illustrated, most of them filled with underlinings and pencil notes on the margin; there were also a couple in Portuguese and some other in Italian. They were of all sorts of literary genres, although I could spot a common subject. I asked the owner where he had gotten these boxes. They had belonged to a man that had recently died; that is all he knew. He had bought them at an auction, along with other private libraries and lots from all over the place. I asked him to give me a price, and I took the whole lot home. For several weeks, I left the books in their boxes, forgotten in a room, as my job prevented me from going through them. When I finally found the time to exhume them, I found, scattered among several volumes, a manuscript in the form of correspondence: ten long letters, written in tight, minuscule handwriting on double-sided paper (the same handwriting responsible for the notes in the book margins). The last of these letters dated from just three months before my casual visit to the old man's bookshop, so the author must have written it right before he passed away. (He'd still have time, however, for a mysterious trip abroad. But we'll talk more about that later on.) Regarding his identity, my inquiries proved fruitless (the signature at the end of each letter was unreadable). The only biographical information we have, then, is what the author tells us throughout the manuscript: not much beyond his marital status as a widower, and his wife's, who is the recipient and leitmotif of the letters, first name: Blanca. "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 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.