What can fashionable ideas, blind faith, or pure fantasy possibly have to do with the scientific quest to understand the universe? Quite a bit, argues Nobel Laureate Roger Penrose, who suggests that researchers working at the extreme frontiers of physics are just as susceptible to these forces as anyone else. While allowing that subjective ideas can sometimes be productive in physics, they may be leading today's researchers astray in three of the field's most important areas—string theory, quantum mechanics, and cosmology. Reflecting on his long career, Penrose also owns up to some of his most unusual theories, including one dubbed "conformal crazy cosmology."