We combine observations of a partial filament eruption on 11 December 2011 with a simple line-current model to demonstrate that including mass is an important next step for understanding solar eruptions. Observations from the Solar Terrestrial Relations Observatory-Behind (STEREO-B) and the Solar Dynamics Observatory (SDO) spacecraft were used to remove line-of-sight projection effects in filament motion and correlate the effect of plasma dynamics with the evolution of the filament height. The two viewpoints enable the amount of mass drained to be estimated, and an investigation of the subsequent radial expansion and eruption of the filament. We use these observational measurements to constrain a line-current model and quantitatively demonstrate the important role that the presence and draining of mass has in the lead-up to solar eruptions. Specifically, we show that the balance of magnetic and gravitational forces acting on the line-current is increasingly sensitive to mass perturbations as it approaches its loss-of-equilibrium. Finally, we conclude that the eruption of the observed filament was restrained until 15% of the mass had drained from the structure.