This New York Times story related to the debate over the meritless theory that autism is caused by mercury exposure raises a broader issue that has been troubling me lately: why do people develop such strong beliefs about particular things that no amount of evidence, or for that matter common sense, is enough to sway them? Do take one particularly egregious example example, why do millions of people around he world people believe that water can retain a "memory" of materials diluted away, such that homeopathic remedies that may contain no detectable trace of the supposed "medicine" work?