Unless some theory of eternal recurrence is true, every macroscopic object and event is unique. But if we are to use past experience as a guide to behavior, then we must make judgments about similarities between present objects and events and those experienced in the past. Because the past encounters are not contained in our heads, we must rely on those representations of them that remain. And “experience” should be understood broadly; it includes that acquired directly by an individual during their lifetime, or what has been painfully acquired by other members of the culture and transmitted by some means.