The question, “What is science?” in addition to being philosophically interesting, has important practical and social implications. For example, the National Science Foundation reports that, in 2003, at least 25% of Americans believe that astrology is a true theory.[1] A 2012 Gallup Poll reports that 46% of Americans believe that creationism is a true theory.[2] Given that one acts based upon what one believes, it is reasonable to assume that some Americans act in accordance with astrology or creationism. The consequences of these actions range from silly (only associating with ‘Virgos’) to serious (devaluing scientific exploration in the name of religious dogmatism). Astrology and creationism are widely regarded as non-scientific (or, “pseudo-science”) by the scientifically literate. So, how do scientifically literate individuals make judgments about what is, and what is not, a science? What rational criteria can be proposed that would demarcate, say, physics, astronomy, biology, etc.—the disciplines that we agree are sciences—from pseudo-sciences like astrology and creationism? It turns out that the task of deriving demarcation criteria is difficult: under a bit of rational scrutiny, seemingly satisfactory demarcation criteria turn out to be unsatisfactory. Let’s look at some plausible candidates: