Galileo was somewhat secretive about his important discoveries such as the law of falling bodies, the concept of inertia, and the laws of projectiles. He communicated these ideas, as well as his cosmological concepts, only to those persons like Kepler, who, he knew would accept them or, at the very least, consider them with an open mind. The two corresponded regularly. Galileo first learned of Kepler through a common friend, Paulus Amberger, who delivered Kepler's book, Cosmic Mystery, to Galileo. Kepler, in a letter to his teacher, Maestlin, states that he had sent a copy of his book "to a mathematician Galileus Galileus, as he signs himself."