Borges explains concisely
"[...] We do not exist in the majority of these times; in some you exist, and not I; in others I, and not you; in others, both of us. In the present one, which a favorable fate has granted me, you have arrived at my house; in another, while crossing the garden, you found me dead; in still another, I utter these same words, but I am a mistake, a ghost."
"In every one," I pronounced, not without a tremble to my voice, "I am grateful to you and revere you for your re-creation of the garden of Ts'ui Pên."
"Not in all," he murmured with a smile. "Time forks perpetually toward innumerable futures. In one of them I am your enemy."
(Jorge Luis Borges, "The Garden of Forking Paths", trans. Donald A. Yates)
