What is happiness? We know it when we experience it, but how can we define it?

I heard this on ‘Thought for the Day’ on Radio 4 this morning and thought it came quite close:

“Happiness is not a matter of intensity but of balance, order, rhythm and harmony.”

Thomas Merton (January 31, 1915 – December 10, 1968) was an American Catholic writer and mystic. A Trappist monk of the Abbey of Gethsemani, Kentucky, he was a poet, social activist, and student of comparative religion. In 1949, he was ordained to the priesthood and given the name Father Louis.