The doctrine that there is some feature of living bodies that prevents their nature being entirely explained in physical or chemical terms. This feature may be the presence of a further ‘thing’ (such as a soul), but it may also be simply the emergence of special relations or principles of organization arising from the complexity of the biological organism. The former kind of doctrine envisages Life as a kind of fiery fluid (animal electricity, life force), that needs pouring into an inanimate body in order for it to become alive. Aristotle (De Anima and De Generatione) is the principal source of a more sophisticated vitalism, holding that the life of an animal consists in its psyche, which provides a principle of explanation determining the morphological development of the organism, by a principle of teleological or final causation. In the 19th century the two great exponents of vitalism were Bergson and the biologist Hans Driesch (1867–1941).

Vitalism has been eclipsed by the advance in molecular genetics, and consequent understanding of the development of organisms in terms drawn from normal science, so the consensus amongst philosophers and biologists is that it offers no explanatory principles that the life sciences need. However, there do remain problems in understanding how different levels of description and explanation of one thing, such as those of psychology and those of biology, or those of biology and those of chemistry, relate to each other. see also supervenience.