  The notions of two-dimensional area, Killing fields and matter flux are
introduced in the setting of causal fermion systems. It is shown that for
critical points of the causal action, the area change of two-dimensional
surfaces under a Killing flow in null directions is proportional to the matter
flux through these surfaces. This relation generalizes an equation in classical
general relativity due to Ted Jacobson to the setting of causal fermion
systems.
