There are no rules. One should not try to explain the mystery. It is better just to be receptive, a Leica within easy reach.
Henri Cartier-Bresson’s photographic work began in the 1920s when he traveled to Africa’s Ivory Coast. 
He then began to travel the world and seemed to synchronistically be present at many of mankind’s pivotal events – the Spanish Civil War, the Liberation of Paris, Communist China, Gandhi’s last hours – snapped at what Henri called The Decisive Moment.
Pierre Assouline’s Biography of Henri details his life in a most clear and intimate style. 