 Furthermore, we face the question of how to understand the role of the non-gauge-invariant quantities appearing in the theory, and the problem of how to interpret what M. Redhead calls “surplus structure” (see Redhead, 2003). For further detail on this topic, see Belot (1998), Nounou (2003), Weatherall (2016), and Nguyen, Teh, &amp; Wells (forthcoming). For an approach to these questions using the theory of constrained Hamiltonian systems, see also Earman (2003b) and Castellani (2003, 2004). For an intuitive characterization of gauge symmetry, one that is more general than the Lagrangian and Hamiltonian formulations of theories using which gauge symmetry is usually expressed, see Belot (2008). How best to interpret gauge theories is an open issue in the philosophy of physics. Healey (2007) discusses the conceptual foundations of gauge theories, arguing in favour of a non-separable holonomy interpretation of classical Yang-Mills gauge theories of fundamental interactions. Catren (2008) tackles the ontological implications of Yang-Mills theory by means of the fiber bundle formalism. Useful references are the Metascience review symposium on Healey (2007) (Rickles, Smeenk, Lyre and Healey, 2009), and the “Synopsis and Discussion” of the workshop “Philosophy of Gauge Theory,” Center for Philosophy of Science, University of Pittsburgh, 18–19 April 2009 (available online).