The dominant concept in Aristotle's philosophy of nature is his notion of causation.... The final cause states that each substance has an inherent purpose. Thus there must be a purpose or design in the acorn such that it always grows into an oak tree. This aspect of existence is indicated by the word entelechy; this means the purpose that guides things to develop in one way rather than another.

p. 291 - Before Galileo, The Birth of Modern Science in Medieval Europe (2012)