where ϕ (x) is some type of scalar potential and x is displacement (not distance) in the x direction, the subscripts label two different positions x 1, x 2, and potentials at those points, ϕ 1 = ϕ (x 1), ϕ 2 = ϕ (x 2). In the limit of infinitesimal displacements, the ratio of differences becomes a ratio of differentials :