There is a sense in which we always move from one world to another incommensurable world. We do pass between worlds as we pass from home to office or to classroom. There are no smooth transitions between them, and we do damage by failing to notice the thresholds that we cross. (For example: treating one’s children as one’s students, or vice versa; or treating a family argument as a judicial proceeding.) On such occasions, we, as it were, commit category mistakes. When we transition between the worlds smoothly, when we are correctly handling incommensurable lexicons and situations, we are, in our actual lives, bilingual….