Advocates of traditional antitrust are increasingly called upon to the defend the existing framework. In doing so they face a challenge: the traditional framework is actually quite difficult to explain. The problem is not that modern antitrust involves a lot of advanced economics — though that is also true. The problem is that foundational antitrust concepts like “harm to competition” and the protection of “consumer welfare” are shockingly ill-defined. This essay highlights several of the dormant ambiguities in these concepts, and thus the obstacles that antitrust has set for itself by failing to fully define its terms.