Abstract: For more than seventy years, courts have deferred to reasonable agency interpretations of ambiguous regulations. The Auer principle, as is it is now called, has attracted academic criticism and some skepticism within the Supreme Court. But the principle is entirely correct. In the absence of a clear congressional direction, courts should assume that because of their specialized competence, and their greater accountability, agencies are in the best position to decide on the meaning of ambiguous terms. The recent challenges to the Auer principle rest on fragile foundations, including an anachronistic understanding of the nature of interpretation, an overheated argument about the separation of powers, and an empirically unfounded and logically weak argument about agency incentives, which exemplifies what we call "the sign fallacy."