Reports of sinister clowns in the news have us thinking about creepiness. Why are some things simply scary, and other things genuinely creepy? On today's Please Explain, David Livingstone Smith, Professor of Philosophy at the University of New England, offers some insight in an essay for Aeon called, "A theory of creepiness." He tells us how scientists and researchers have attempted to measure and classify creepiness - from robots that are designed to look like humans (but something isn't quite right), to being put off by physical traits like "unkempt hair, bulging eyes, [and] abnormally long fingers."

David Livingstone Smith is the author of seven books, most recently, Less Than Human: Why We Demean, Enslave and Exterminate Others.