 Although both Einstein and Infeld and Zahar use the same basic argument, they reach slightly different conclusions. Zahar argues that mass-energy equivalence entails that the fundamental stuff of physics is a sort of “I-know-not-what” that can manifest itself as either matter or field. Einstein and Infeld, on the other hand, in places seem to argue that we can infer that the fundamental stuff of physics is fields. In other places, however, Einstein and Infeld seem a bit more cautious and suggest only that one can construct a physics with only fields in its ontology.