Having for thirty years believed and taught the doctrine of phlogiston… I for a long time felt inimical to the new system, which represented as absurd that which I hitherto regarded as sound doctrine; but this enmity… springs only from force of habit… [Black to Lavoisier, 1791]This paper is abstracted from a forthcoming book which defends a particular answer to the question of just what it is that shifts when a paradigm shifts. The claim is that what shifts are habits (...) of mind. And in particular the claim is that the most striking cases of paradigm shift will characteristically turn on a shift in some single, uniquely critical, habit of mind: the barrier. An account of a radical discovery — discovery that prompts the Kuhnian symptoms of incommensurability, so that intuitions that seem irresistible to some seem perverse to their rivals — then characteristically turns on how some individual got past the barrier (escaped the critical habit of mind), while at least for a while others could not do so. (shrink)