 We will begin with a discussion of Bondi and Spurgin’s interpretation (in  Section 2.3.1).  They hold that mass and energy are distinct properties and that there is no such thing as the conversion of mass and energy. We will then discuss Rindler’s interpretation (in  Section 2.3.2).  He maintains that mass and energy are different properties but that genuine conversions of mass and energy are at least permitted by mass-energy equivalence.