The concept-swapping fallacy produces circular definitions and infinite regresses. It is found in theoretical disciplines, as shows the case of Poincaré and Le Roy in the philosophy of mathematics. But it is also found in practical ones. Thus in ethics we find it in the repeated attempts to define the good, by authors as different as Bentham and Mill, on the one hand, or Brentano on the other. And in the philosophy of law we find it in the convoluted ways in which legal positivists attempt to define what is lawful. In all these and many other cases, what is at work is the replacement of a real synthetic judgment by an apparent analytic one.