 The only combination of answers to questions (1) and (2) that is inconsistent is to say that mass and energy are the same property of physical systems but that the conversion of mass into energy (or vice versa) is a genuine physical process. All the other three combinations of answers to questions (1) and (2) are viable options and have been held, at one time or another, by physicists or philosophers as indicated by the examples given in  Table 2.