A natural way to obtain a system of partial differential equations on a manifold is to vary a suitably defined sesquilinear form. The sesquilinear forms we study are Hermitian forms acting on sections of the trivial C n -bundle over a smooth m-dimensional manifold without boundary. More specifically, we are concerned with first order sesquilinear forms, namely, those generating first order systems. Our goal is to classify such forms up to GL(n, C) gauge equivalence. We achieve this classification in the special case of m = 4 and n = 2 by means of geometric and topological invariants (e.g. Lorentzian metric, spin/spin c structure, electromagnetic covector potential) naturally contained within the sesquilinear form -a purely analytic object. Essential to our approach is the interplay of techniques from analysis, geometry, and topology.