The output of any general continuous-time linear system is related to the input by an integral which may be written over a doubly infinite range because of the causality condition: y ( t ) = ∫ ∫ − − ∞ ∞ t h ( t , t ′ ) x ( t ′ ) d t ′ = ∫ ∫ − − ∞ ∞ ∞ ∞ h ( t , t ′ ) x ( t ′ ) d t ′