The most important and most controversial aspect of Kuhn’s theory involved his use of the terms “paradigm shift” and “incommensurability.” That the scientific terms of one paradigm are incommensurable with the scientific terms of the paradigm that replaces it. A revolution occurs. One paradigm is replaced with another. And the new paradigm is incommensurable with the old one. He made various attempts to define it — changing and modifying his definitions along the way. In the 1962 edition of “Structure” incommensurability was likened to a Gestalt-flip. Presumably, it was about how we see the world.