To his friends, he was always something of an enigma.
one of the great enigmas of our time
Despite the enigma of the singularity, the big bang theory is unquestionably one of the most successful ideas in the history of science. —Tom Yulsman, Astronomy, September 1999
Many scholars call it the ultimate enigma of the Nazi era, indeed perhaps of modern history: How did the Holocaust happen? And why in Germany? —Ralph Blumenthal, New York Times, 10 Jan. 1998
Indeed, the bond that in every individual connects the physiological life and the psychic life—or better the relation existing between the contingence of an individual and the free spirit that assumes it—is the deepest enigma implied in the condition of being human, and this enigma is presented in its most disturbing form in woman. —Simone de Beauvoir, translated and edited by H.M. Parshley, The Second Sex, 1953