In the Horizon of the Infinite Friedrich Nietzsche

We have left the land and have gone aboard ship! We have broken down the bridge behind us, nay, more, the land behind us! Well, little ship! look out! Beside thee is the ocean; it is true it does not always roar, and sometimes it spreads out like silk and gold and a gentle reverie. But times will come when you wilt feel that it is infinite, and that there is nothing more frightful than infinity. Oh, the poor bird that felt itself free, and now strikes against the walls of this cage! Alas, if home sickness for the land should attack thee, as if there had been more freedom there, and there is no "land" any longer!