 Hume identifies three principles of association: resemblance, contiguity in time and place, and causation. When someone shows you a picture of your best friend, you naturally think of her because the picture resembles her. When you’re reminded of something that happened in the 1960s—miniskirts, for example—you may think of the Vietnam War, because they are temporally contiguous. Thinking of Sausalito may lead you to think of the Golden Gate Bridge, which may lead you to think of San Francisco, since they are spatially contiguous. Causality works both from cause to effect and effect to cause: meeting someone’s father may make you think of his son; encountering the son may lead you to thoughts of his father. 