  We define a new measure of causation from a fluctuation-response theorem for
Kullback-Leibler divergences, based on the information-theoretic cost of
perturbations. This information response has both the invariance properties
required for an information-theoretic measure and the physical interpretation
of a propagation of perturbations. In linear systems, the information response
reduces to the transfer entropy, providing a connection between Fisher and
mutual information.
