  I analyze the meaning of mass in Newtonian mechanics. First, I explain the
notion of primitive ontology, which was originally introduced in the philosophy
of quantum mechanics. Then I examine the two common interpretations of mass:
mass as a measure of the quantity of matter and mass as a dynamical property. I
claim that the former is ill-defined, and the latter is only plausible with
respect to a metaphysical interpretation of laws of nature. I explore the
following options for the status of laws: Humeanism, primitivism about laws,
dispositionalism, and ontic structural realism.
