Karl Raimund Popper’s The Logic of Scientific Discovery, one of the most important books ever written on the philosophy of science, begins with the problem of induction. An inference is inductive, Popper explains, if it moves from a singular statement (roughly, a statement whose subject term refers to some particular concrete thing) to one or more universal statements (roughly, statements whose subject terms refer to all the members of a class of things). In science, such inferences occur when one passes from descriptions of particular experimental results to hypotheses or theories alleged to be justified by these results. “All observed swans have been white” sums up a set of particular statements that report observations of concrete particular items. “All swans are white” expresses a universal statement one might inductively infer from that summary.