 Wharton (2010a; see also Wharton 2007, 2010b, 2013, 2016, 2018; Wharton, Miller, &amp; Price 2011; Evans, Price, &amp; Wharton 2013) proposes a “novel interpretation of the Klein-Gordon equation” for a neutral, spinless relativistic particle. The account is a retrocausal picture based on Hamilton’s principle and the symmetric constraint of both initial and final boundary conditions to construct equations of motion from a Lagrangian, and is a natural setting for a perspectival interventionist account of causality. Wharton treats external measurements as physical constraints imposed on a system in the same way that boundary constraints are imposed on the action integral of Hamilton’s principle; the final measurement does not simply reveal preexisting values of the parameters, but constrains those values (just as the initial boundary condition would). Wharton’s model has been described as an “all-at-once” approach, since the dynamics of physical systems between an initial and final boundary emerges en bloc as the solution to a two-time boundary value problem.