Life is the capacity in matter, formed of one or more units called cells, for processes such as cell signaling, homeostasis, metabolism, cell growth, adaptation, response to stimuli, and reproduction. All life eventually reaches a state of death. Many philosophical definitions of living systems have been proposed, such as self-organizing systems. Defining life is further complicated by viruses, which replicate only in host cells, and the possibility of extraterrestrial life, which could be very different from life on Earth. Life exists all over the Earth in air, water, and soil, with many ecosystems forming the biosphere. Some of these are harsh environments occupied only by extremophiles. The life in a particular ecosystem is called its biota.