Each Christmas, the Manhattan literary agent John Brockman gives his pals a "riddle me this."
A year ago he brain-teased: "What do you believe is true even though you cannot prove it?" And this time: "What is your dangerous idea?" Brockman's challenge is noteworthy because his buddies include many of the world's greatest scientists: Freeman Dyson, David Gelertner, J. Craig Venter, Jared Diamond, Brian Greene. Yet their ideas, delineated in brief and engaging essays, are not just for tech-heads. The 119 responses Brockman received to the most recent question -- posted at www.edge.org  -- are dangerous precisely because they so often stray from the land of test tubes and chalkboards into the realms of morality, religion and philosophy.