  One problem with this theory is that no two classes can have the same members, while it does not seem that properties with the same instances need be the same. So there is no guarantee that the identification of properties with classes is correct. And even if correct, the identification is clearly not necessarily correct. Furthermore, if every F is a G and vice versa, the theory forces us to say that what makes something F is the same as what makes it G. But while every F might be a G and vice versa, it does not follow that what makes things F is the same as what makes them G.