 The second core Russellian monist thesis, realism about quiddities, states that there are (instantiated) properties of precisely the sort about which, according to structuralism about physics, physical theory is silent: properties that underlie the spatiotemporal structure physical theory describes. Those properties categorically ground the most basic physical dispositions that physics describes, in the way a ball’s spherical shape categorically grounds its disposition to roll when pushed. These underlying properties are often called quiddities (Lewis 2009; Chalmers 2012). They are also called inscrutables (Montero 2010). The latter designation is meant to indicate something on which many proponents of realism about quiddities agree: we know little about quiddities beyond the theoretical roles they are supposed to play.