In Chap. 3 we looked at the basic arguments that nonliberal authors have put forward in favour of paternalism: the perfectionist argument, the utilitarian-organicist argument, and the very popular slippery slope argument. In recent years, however, a paternalistic argument has been advanced by authors claiming to be liberal, an argument based on the findings of cognitive science, that is, on the idea that human beings are prone to make numerous cognitive errors in their everyday reasoning. Let us see why cognitive errors may pose a problem for a liberal theory. In reasoning vitiated by widespread cognitive errors or biases or by common framing errors, our judgments would seem to have the same effect as false beliefs, widely regarded as symptoms that we are insufficiently rational or, at any rate, that our decisions are not based on a knowledge of the relevant facts. If a decision is in fact based on a false belief, the means we identify to achieve the goal pursued through that decision could be unsuitable. Similarly, if we commit a relevant cognitive error—especially one that is very difficult for us to detect and hence avoid, unless we are cognitive psychologists—then our decision could be unsuitable for achieving our desires.