Translated by Ku Hung Ming
http://en.wikipedia.org/wiki/Gu_Hongming

When the passions, such as joy, anger, grief and pleasure have not awakened, that is out true self (中) or moral being.  When these passions awaken and each and all attain due measure and degree, that is the moral order(和). Our true self or moral being is the great reality (大本 lit. great root) of existence, and moral order is the universal law(达道) in the world.

- The Conduct of Life

p.s. Chinese original
喜，怒，哀，乐之未发，谓之中。发而皆中节，谓之和。中也者， 天下之大本也。和也者，天下之达道也。