In philosophy, especially that of Aristotle, the golden mean is the desirable middle between two extremes, one of excess and the other of deficiency. For example courage, a virtue, if taken to excess would manifest as recklessness and if deficient as cowardice.
To the Greek mentality, it was an attribute of beauty. Both ancients and moderns realized that there is a close association in mathematics between beauty and truth. The poet John Keats, in his Ode on a Grecian Urn, put it this way:
“Beauty is truth, truth beauty,” — that is all
Ye know on earth, and all ye need to know.
The Greeks believed there to be three ‘ingredients’ to beauty: symmetry, proportion, and harmony. This triad of principles infused their life. They were very much attuned to beauty as an object of love and something that was to be imitated and reproduced in their lives, architecture, education (Paideia) and politics. They judged life by this mentality.