The determination of stability for a scalar-pressure toroidal plasma is formulated by means of an energy variational principle in which the geometric problem of variation of the flux-surface shapes is separated from the variation of the flux profile. By means of successive minimizations, holding the surface shapes and the flux profiles fixed, the potential energy is written as a one-dimensional integral over a function of pressure, differential flux, and the specific-inductance matrix. The latter depends only on the geometry of the surfaces. Stability conditions are derived by varying the flux profile and the geometry (specific inductance). Also, a non-linear stability condition is derived for the special case of a finite geometric distortion of the flux surfaces while holding the flux profiles fixed. Exact representations for the specific inductance are calculated for straight cylinders, axisymmetric toruses, and helically symmetric configurations, all with arbitrary cross sections.