As an example, consider the current J ( x , t ) that is driven in response to an electric field E ( x , t ) , which is varying in space (x) and time (t). Simplified laws such as Ohm's law would say that these are directly proportional to each other, J = σ σ E , but this breaks down if the system has memory (temporal dispersion) or spreading (spatial dispersion). The most general linear response is given by: