What is your Dangerous Idea?

The history of science is replete with discoveries that were considered socially, morally, or emotionally dangerous in their time; the Copernican and Darwinian revolutions are the most obvious. What is your dangerous idea? An idea you think about (not necessarily one you originated) that is dangerous not because it is assumed to be false, but because it might be true?

This question, and the answers of various intellectuals, are available on Edge.
(scroll down halfway for the beginning of the answers)

For those who love intellectual stimulation, this is a great idea.
(there are 119 original essays)