The physical sciences include astronomy, physics, chemistry, and the earth sciences, but the last of these sciences is quite unlike the other three. Whereas the objects of study in physics and chemistry often seem abstract to the uninitiated and astronomy is concerned with faraway planets and other bodies, the earth sciences are devoted to things that are both concrete and immediate. The focus of study for the earth sciences is literally underneath our feet, a planet at once vast and tiny, a world that (as far as we know) stands alone in the universe as the sole supporter of intelligent life. The earth sciences also differ from other disciplines in that their boundaries are not always defined clearly. The study of Earth is a multifarious array of specialties that includes a range of geologic, hydrologic, and atmospheric sciences that overlap with the other physical sciences, biology, and even the social sciences.