 The parallel with the first formulation of the Categorical Imperative—“act only in accordance with that maxim through which you can at the same time will that it become a universal law” (4:421)—hardly needs spelling out. Kant now says: think only in accordance with that maxim that could be a universal  law.[23]  Differently put: thinking is an activity, and if the Categorical Imperative is indeed “categorical” then it applies to all our activities.