Abstract: This article discusses Mark McKenna’s Testing Modern Trademark Law’s Theory of Harm as an important step forward in challenging trademark expansionism, going back to basics and asking us to assess for truth value several propositions that now seem so self-evident to lawyers and judges as to not require any empirical support at all. Like McKenna, the author believes that if the law looked for the evidence behind present axioms of harm, it would not find much there. McKenna and the author share an interest in empirical evidence on marketing and a desire to bring its insights to trademark law. But how did today’s theories of harm resulting from any kind of confusion, even confusion over unrelated goods, become “common sense” to judges, particularly when Mark Lemley asserts that modern Lanham Act jurisprudence represents “the death of common sense”? McKenna has traced the history of the harm argument in the courts, but figuring out exactly why that argument became persuasive—so persuasive, in fact, that courts now take it as factual beyond debate—is difficult. Common sense, often employed in legal reasoning, tends to hide its empirical and normative judgments in ways that make analysis difficult. McKenna, like other trademark restrictionists, tries to push back against today’s common sense with facts that will, as a whole, constitute a new (or even an old) common sense. His project limits trademark more tightly to protection against competition, with extensions only where necessary to avoid actual harm. The author briefly examines the role of common sense inferences, and how courts might deploy these inferences differently in trademark litigation.