 Putting the same point somewhat more formally, any linear differential operator L associated with an inhomogeneous differential equation \(\mathrm{L}y=f(x)\) and with constant coefficients possesses a fundamental solution or Green’s function G, which is a solution to the inhomogeneous differential equation \(\mathrm{LG}=\delta (x),\) where \(\delta(x)\) is the delta function, a generalized function that is zero everywhere except at \(x=0.\) The Green’s function tells us what the contribution of introducing a point-disturbance or perturbation into a system at \((x', t')\) is to the state of the system at some other point \((x, t).\) The overall response of a system at \((x, t)\) to multiple perturbations is calculated by summing or integrating over all point-like disturbances. The most general solution to a theory is given by adding to this response to disturbances a solution to the homogenous equation \(\mathrm{L}y=0.\)