What are physical quantities, and in particular, what makes them quantitative? This book articulates and defends an original answer to this important, insufficiently understood question through the novel position of substantival structuralism. This position argues that quantitativeness is an irreducible feature of attributes, and quantitative attributes are best understood as substantival structured spaces. The book first explores what it means for an attribute to be quantitative, and what metaphysical implications a commitment to quantitative attributes has. It then sets the stage to address the metaphysical and ontological consequences of the existence of quantitative attributes.

"Wolff is more persuasive when critiquing comparativism's pull towards fundamentalism about quantities' ordering relations." - Toby Friend, Philosophical Quarterly

"this excellent contribution ... is a mustmust-read for anyone who is interested in scientific measurement or scientific representation in general and will no doubt be the reference point for the discussions on this topic for years to come." - Derek Lam, Metascience