It is still a challenge for scientists and philosophers to define life in unequivocal terms. Defining life is difficult—in part—because life is a process, not a pure substance. Any definition must be sufficiently broad to encompass all life with which we are familiar, and it should be sufficiently general that, with it, scientists would not miss life that may be fundamentally different from life on Earth. Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes (i.e., living organisms) from those that do not, either because such functions have ceased (death), or else because they lack such functions and are classified as inanimate. Biology is the science concerned with the study of life.

Living organisms undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations. More complex living organisms can communicate through various means. A diverse array of living organisms (life forms, approximtely 8.4 million forms of life) can be found in the biosphere on Earth, and the properties common to these organisms—plants, animals, fungi, protists, archaea, and bacteria—are a carbon- and water-based cellular form with complex organization and heritable genetic information. In philosophy and religion, the conception of life and its nature varies. Both offer interpretations as to how life relates to existence and consciousness, and both touch on many related issues, including life stance, purpose, conception of a God or Gods, a Soul or an afterlife.


Hylomorphism is the theory (originating with Aristotle (322 BC)) that all things are a combination of matter and form. Aristotle was one of the first ancient writers to approach the subject of life in a scientific way. Biology was one of his main interests, and there is extensive biological material in his extant writings. According to him, all things in the material universe have both matter and form. The form of a living thing is its Soul (Greek psyche, Latin anima). There are three kinds of Souls: the “vegetative soul” of plants, which causes them to grow and decay and nourish themselves, but does not cause motion and sensation; the “animal soul” which causes animals to move and feel; and the “rational soul” which is the source of consciousness and reasoning which (Aristotle believed) is found only in humans. Each higher soul has all the attributes of the lower one. Aristotle believed that while matter can exist without form, form cannot exist without matter, and therefore the soul cannot exist without the body.

Vitalism is the belief that the life-principle is essentially immaterial. This originated with Stahl (17th century), and held sway until the middle of the 19th century. It appealed to philosophers such as Henri Bergson, Nietzsche, Wil- helm Dilthey, anatomists like Bichat, and chemists like Liebig.

Vitalism underpinned the idea of a fundamental separation of organic and inorganic material, and the belief that organic material can only be derived from living things. This was disproved in 1828 when Friedrich Wöhler prepared urea from inorganic materials. This so-called Wöhler synthesis is considered the starting point of modern organic chemistry. It is of historical significance because for the first time an organic compound was produced from inorganic reactants.

Later, Helmholtz, anticipated by Mayer, demonstrated that no energy is lost in muscle movement, suggesting that there were no vital forces necessary to move a muscle. These empirical results led to the abandonment of scientific interest in vitalistic theories, although the belief lingered on in non-scientific theories such as homeopathy, which interprets diseases and sickness as caused by disturbances in a hypothetical vital force or life force.