An often elusive doctrine, ultimately inspired by Aristotle, which holds that the phenomenon of life cannot be fully explained in purely material terms, but that it is something non-material in living organisms that differentiates them from inanimate bodies. Following the considerable advances in biological science in the 19th century, vitalists opposed the doctrine of “mechanism”, which claimed that the activities that characterize all living things can be accounted for in terms of physico-chemical processes. Such activities were understood as due to entelechies defined as autonomous, mind like, non-spatial entities that exercise control over the course of organic process.