We began this book in Chap. 1 by tracing the gradual development, from Clausius and Boltzmann in the 19th century to current developments in the 21st, of the idea that physical interactions can all, without exception, be viewed as implementing information transfer. Provided the interaction is finite in space, time, and energy, the information transferred is finite. The symmetry of the Hamiltonian— \(H_{SE} = H_{ES}\) , Newton’s Third Law, or the discrete nature of finite holographic screens—guarantees that the information transfer is also symmetrical. All separable, multi-component physical systems are, therefore, distributed information-flow systems. Indeed they are communication systems. In the language of the FEP, they are self-organizing active-inference agents.