Computationalism is the view that computation, an abstract notion lacking semantics and real-world interaction, can offer an explanatory basis for cognition. This paper argues that not only is this view correct, but that computation, properly understood, would seem to be the only possible form of explanation! The argument is straightforward: To maximise their chances of success, cognitive agents need to make predictions about their environment. Models enable us to make predictions, so agents presumably incorporate a model of their environment as part of their architecture. Constructing a model requires instantiating a physical "device" having the necessary dynamics. A program and the computation it defines comprise an abstract specification for a causal system. An agent's model of its world (and presumably its entire mechanism) can thus be described in computational terms too, so computationalism must be correct. Given these interpretations, the paper refutes arguments that purport to show that everything implements every computation (arguments which, if correct, would render the notion of computation vacuous.) It then goes on to consider how physical systems can "understand" and interact intelligently with their environment, and also looks at dynamical systems and the symbolic vs. connectionist issue.