Thus far, I have reviewed the first four chapters of Thomas Bingham’s book Rule of Law, pointing out some holes and flaws in Bingham’s work, such as the problem of vague or conflicting laws (see here) and the problem of hard cases (here). Today, I will point out a common but egregious logical fallacy that appears in Chapter 5, which is titled “Equality before the law”. To the point, Judge Bingham begins this chapter with the following legal formula: “The laws of the land should apply equally to all, save to the extent that objective differences justify differentiation.” The more general idea that Bingham is trying to defend is the idea that like cases should be treated alike. Alas, I hate to be that guy, but without some external criterion (or set of criteria) for deciding when two cases are sufficiently similar or for deciding when it’s okay to treat people differently, the principle of equality, standing alone, does absolutely no substantive work in the real world. Why not? Because all laws, by definition, make classifications! As a result, as Peter Weston explained many years ago in his classic Harvard Law Review article “The Empty Idea of Equality“, we will always need some external theory, principle, or rule–i.e. a theory, principle, or rule separate from the idea of equality itself–to figure out which classifications are justified and which are not. Stated formally, Bingham’s formula is tautological, for what he is really saying is that the law must apply to everyone equally except when it doesn’t have to.