I critically examine the plausibility of Kant’s conception of rationalising, a form of self-deception that plays a crucial role for Kant’s moral psychology and his conception of the functions of critical practical philosophy. The main problem I see with Kant’s conception is that there are no theory-independent criteria to determine whether an exercise of rational capacities constitutes rationalising. Kant believes that rationalising is wide-spread and he charges the popular philosophers and other ethical theorists with rationalising. Yet, his opponents could, in (...) turn, charge him with rationalising and some theorists, namely Act-Consequentialists, seem to be in an even stronger position to charge Kant with rationalising than vice versa. In response, I propose standards that do not assume a specific normative theory and that become apparent when we look at clear-cut abuses of rationality. These standards of minimally decent reasoning can help us diagnose rationalising. I develop these standards by looking at inadequate uses of rational capacities that should strike us as problematic regardless of the specific ethical theory we adopt. I emphasise that even an abuse of rational capacities can yield true results and that we can never tell from a single judgement that someone rationalises. Rather, we must look for patterns. (shrink)