Computation by natural systems defined

An explicit measure of the computation performed by general systems (electronic circuits, neurons, mechanical devices, etc.) is defined. We propose that the deeper nature of computation, and thus of any measure of computation, is in its reduction of complexity. The latter we understand as the "obstruction against prediction", experienced by an observer. We demonstrate the applicability and usefulness of this concept in different examples, which include some of the most studied families of dynamical systems. The measure can also be computed for higher-dimensional and experimental systems.