 There are many properties that have to do with how massive things are: they include determinate mass properties like being 17 grams in mass; more general properties like having mass; relations between objects like being more massive than; relations among properties like being a greater mass than; and many more. But what about mass itself? Is it one of the aforementioned properties? Some authors have suggested that the answer is no: things like mass and density—and perhaps also shape, athletic ability, etc.—belong to a sui generis ontological category of “quantities” or “magnitudes”. Each of these items is associated with certain values or degrees, e.g. three grams or three grams per cubic centimeter; these may belong to some further sui generis category. An object can “have” a magnitude “to” a degree. But the magnitude mass is not the binary relation object \(x\) has mass to degree \(y\).