Abstract
The current induced in the vacuum by an external field is a nonlinear functional of the latter when one goes beyond the first approximation. It is demonstrated that when this induced current is suitably defined it will satisfy the causality requirement of being zero before the external field is switched on. From this causality principle an exact dispersion relation is deduced. This relaxation of the linearity restriction in deriving a dispersion relation from causality implies that it may be possible to drop the linearity restriction in discussing the equivalence of causality and dispersion relations for other systems.
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