Full Idea
It is definition of essence that makes things understandable.
Gist of Idea
Definition of essence makes things understandable
Source
Thomas Aquinas (De Ente et Essentia (Being and Essence) [1267], p.92)
Book Reference
Aquinas,Thomas: 'Selected Philosophical Writings', ed/tr. McDermott,Timothy [OUP 1993], p.92
A Reaction
The aim of philosophy is understanding, which is achieved by successful explanation. I totally agree with this Aristotelian view, so neatly summarised by Aquinas.