The idea that a balanced life is an ideal life has persisted for centuries.  You can find it in the philosophies of every era of history from the ancients to new age proponents—and throughout the writings of Baron Pierre de Coubertin.  He believed that the balanced life was a byproduct that was manifest in those who achieved fitness physically, mentally and emotionally.  In ‘Olympic Letter VII: The Recipe for Becoming Olympic,’ which he published in the Olympic Review in 1918, he made the point that balance would be stifled by too much caution but arrive through the practice of complementary disciplines. 

“In life, balance is a result, not a goal, a reward rather than something to be sought out. It is not achieved by taking every possible precaution, but by alternating one's efforts.”