In a letter to L`Hôpital in 1695 Leibniz raised the following question: "Can the meaning of derivatives with integer order be generalized to derivatives with non-integer orders?" L`Hôpital was somewhat curious about that question and replied by another question to Leibniz: "What if the order will be 1/2?" Leibniz in a letter dated September 30, 1695 — the exact birthday of the fractional calculus! — replied: "It will lead to a paradox, from which one day useful consequences will be drawn."