1. Consider a circle. It has both a radius and a circumference. There is obviously a real distinction between the properties having a radius and having a circumference. This is not because, when confining ourselves to circles,1 having a radius can ever exist apart from having a circumference. A real distinction does not depend on that. Descartes thought that a real distinction between x and y meant that x could exist without y or vice versa, if only by the power of God. But Descartes was wrong. Separable existence is a sufficient but not necessary condition of there being a real distinction. The difference between a real and a conceptual distinction derives from medieval philosophy. Aquinas, for one, held that things can be really distinct even though not separable (the form and matter of a material substance or its essence and existence, for example).2 For a merely conceptual distinction between x and y to exist, it is necessary for the distinction to exist in thought only. There is only a conceptual distinction between an upward slope and a downward slope, or between a glass’s being half empty and half full. Not only are the members of such pairs inseparable (whether by God or in any other way), but there is no real distinction between them. There is no numerical distinctness between the entities or qualities between which there is only a conceptual distinction. To this extent alone is Galen Strawson (2008) correct.3 But when it comes to...