Although some spoke out in favor of moderate inequality as a form of incentive, others warned against excessive levels of inequality, including Robert J. Shiller, (who called rising economic inequality "the most important problem that we are facing now today"), former Federal Reserve Board chairman Alan Greenspan, ("This is not the type of thing which a democratic society – a capitalist democratic society – can really accept without addressing"), and President Barack Obama (who referred to the widening income gap as the "defining challenge of our time").