Full Idea
In the traditional terminology, function follows essence. Essence just is the principle from which flows the characteristic behaviour of a thing.
Gist of Idea
Essence is the source of a thing's characteristic behaviour
Source
David S. Oderberg (Real Essentialism [2007], 2.1)
Book Reference
Oderberg,David S.: 'Real Essentialism' [Routledge 2009], p.23
A Reaction
Hence essence must be identified if the behaviour is to be explained, and a successful identification of essence is the terminus of our explanations. But the essences must go down to the micro-level. Explain non-characteristic behaviour?