In this article, Professor Deborah A. Geier brings to bear the insights of Professor Edward J. McCaffery, regarding the interaction of cognitive theory and the tax world, to the flat tax proposal. The article explores how the perceptual biases described by Professor McCaffery might affect both taxpayers' impressions of the contours of the proposed tax base and their behavioral reponses to the same incentive. The author warns that any errors in her application of Professor McCaffery's work to the flat tax are entirely her own.