<P> Democritus (460 BC) thought that the essential characteristic of life is having a soul (psyche). Like other ancient writers, he was attempting to explain what makes something a living thing . His explanation was that fiery atoms make a soul in exactly the same way atoms and void account for any other thing . He elaborates on fire because of the apparent connection between life and heat, and because fire moves . </P> <P> Plato's world of eternal and unchanging Forms, imperfectly represented in matter by a divine Artisan, contrasts sharply with the various mechanistic Weltanschauungen, of which atomism was, by the fourth century at least, the most prominent...This debate persisted throughout the ancient world . Atomistic mechanism got a shot in the arm from Epicurus...while the Stoics adopted a divine teleology...The choice seems simple: either show how a structured, regular world could arise out of undirected processes, or inject intelligence into the system . </P> <P> The mechanistic materialism that originated in ancient Greece was revived and revised by the French philosopher René Descartes, who held that animals and humans were assemblages of parts that together functioned as a machine . In the 19th century, the advances in cell theory in biological science encouraged this view . The evolutionary theory of Charles Darwin (1859) is a mechanistic explanation for the origin of species by means of natural selection . </P> <P> Hylomorphism is a theory first expressed by the Greek philosopher Aristotle (322 BC). The application of hylomorphism to biology was important to Aristotle, and biology is extensively covered in his extant writings . In this view, everything in the material universe has both matter and form, and 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 man . Each higher soul has all of the attributes of the lower ones . Aristotle believed that while matter can exist without form, form cannot exist without matter, and that therefore the soul cannot exist without the body . </P>

What is the definition of life processes in biology