 It is unclear, however, how broadly applicable Dowe’s account is. Classical electrodynamics satisfies energy-momentum conservation. Yet while charged particles are associated with world lines, the electromagnetic field, with which charged particles interacts, cannot be associated with a world line along which energy is conserved. As Marc Lange (2002) has argued, it is problematic to think of the electromagnetic field energy as a kind of “stuff” that flows in a uniquely identifiable manner among different field-regions. Thus, Dowe’s conserved quantity account appears to be designed for an ontology of discrete objects and it is unclear how the account might be extended to cover field theories as well.