 According to Kuhn, he discovered incommensurability as a graduate student in the mid to late 1940s while struggling with what appeared to be nonsensical passages in Aristotelian physics (Kuhn 2000 [1989], 59–60). He could not believe that someone as extraordinary as Aristotle could have written them. Eventually patterns in the disconcerting passages began to emerge, and then all at once, the text made sense to him: a Gestalt switch that resulted when he changed the meanings of some of the central terms. He saw this process of meaning changing as a method of historical recovery. He realized that in his earlier encounters, he had been projecting contemporary meanings back into his historical sources (Whiggish history), and that he would need to peel them away in order to remove the distortion and understand the Aristotelian system in its own right (hermeneutic history). For instance, when he encountered the word “motion” in Aristotle (the standard translation of the Greek kinesis), he was thinking in terms of the change of position of objects in space (as we do today). But to get more closely at Aristotle’s original usage, he had to expand the meaning of motion to cover a much broader range of phenomena that include various other sorts of change, such as growth and diminution, alternation, and generation and corruption, making an objects motion in space (displacement or ‘locomotion’) just a special case of motion. Kuhn realized that these sorts of conceptual differences indicated breaks between different modes of thought, and he suspected that such breaks must be significant both for the nature of knowledge, and for the sense in which the development of knowledge can be said to make progress. Having made this discovery, Kuhn changed his career plans, leaving theoretical physics to pursue this strange phenomenon. Some fifteen years later the term ‘incommensurable’ first appears in his classic The Structure of Scientific Revolutions (1962).