 Should we then adopt this structuralist account, and say that we have removed the mysteries of mathematics? As noted above, Lewis is uncharacteristically equivocal on this point, and seemed to change his mind about whether structuralism was, all things considered, a good or a bad deal. His equivocation comes from two sources. One worry is that when we work through the details, some of the mysteries of set theory seem to have been relocated rather than solved. For instance, if we antecedently understood the singleton function, we might have thought it could be used to explain why the set theoretic universe is so large. Now we have to simply posit a very large universe. Another is that the proposal is in some way revisionary, since it takes ordinary mathematical talk to be surreptitiously quantificational. Parts of Classes contains some famous invective directed against philosophers who seek to overturn established science on philosophical grounds.