Martin Hellman: When the risk is highly uncertain, how do you determine who’s right? Is the risk of nuclear deterrence failing acceptable? Former secretary of defense James Schlesinger thought so. In a 2009 interview, he stated that the U.S. would need a strong nuclear deterrent “more or less in perpetuity.” In contrast, former secretary of defense Robert McNamara stated in the 2003 documentary Fog of War that “the indefinite combination of human fallibility and nuclear weapons will destroy nations.” So, is the risk of failure acceptable or unacceptable? When nuclear risk is stated in generalities, it is difficult to determine who is right.