 Logic, according to Stebbing, is not an art, understood as “a set of rules the learning of which may fit some one to do something”, but a science—the science of possible forms (1933b: 473–4). Insofar as the logical forms that the science of logic discovers are norms of thinking, logic can be described as a normative science. But its normativity is not its distinguishing feature, she argues, but is merely a by-product of the fact that norms are what are discovered in logic. She offers an account of the development of logic as the science of form in the final chapter of the book, tracing it from its roots in Aristotle’s Prior Analytics to Whitehead and Russell’s Principia Mathematica, in which it is finally shown, she claims, that “Demonstration as such is purely formal” (1933b: 488).