Nature is in form rather than in matter; what is potentially flesh or bone has not yet acquired its own nature, and a thing is more what it is when it has attained to fulfilment.
 Bertrand Russell, A History of Western Philosophy (1945). copy citation

Context

“if they have an internal principle of this kind. The phrase “according to nature” applies to these things and their essential attributes. (It was through this point of view that “unnatural” came to express blame.) Nature is in form rather than in matter; what is potentially flesh or bone has not yet acquired its own nature, and a thing is more what it is when it has attained to fulfilment. This whole point of view seems to be suggested by biology: the acorn is “potentially” an oak. Nature belongs to the class of causes which operate for the sake of something This leads to a discussion of the view that nature works of necessity, without purpose, in connection with which Aristotle discusses the survival of the fittest, in the form taught by Empedocles.” source