Abstract

To state an important fact about the photon, physicists use such expressions as (1) “the photon has zero (null, vanishing) mass” and (2) “the photon is (a) massless (particle)” interchangeably. Both (1) and (2) express the fact that the photon has no non-zero mass. However, statements (1) and (2) disagree about a further fact: (1) attributes to the photon the property of zero-masshood whereas (2) denies that the photon has any mass at all. But is there really a difference between saying that something has zero mass (charge, spin, etc.) and saying that it has no mass (charge, spin, etc.)? Does the distinction cut any physical or philosophical ice? I argue that the answer to these questions is yes. Put briefly, the claim of this paper is that some zero-value physical quantities are not mere “privations”, “absences” or “holes in being”. They are respectable properties in the same sense in which their non-zero partners are. This, I will show, has implications for the debate between two rival views of the nature of property, dispositionalism and categoricalism.