The observer has so far avoided comprehensive integration into the formalism of physics, even if some observer related effects were found to be indissociable from quantum mechanics about a century ago. Here, we propose a theory of the observer sufficiently comprehensive to be suitable for use as the foundation to all physics. Our proposal significantly simplifies modern physics, reducing it only to a single axiom along with a few pages of mathematical derivation. Specifically we show that our

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... eory uniquely entails quantum field theory, general relativity as well as the indispensable quantum gravity, in addition to providing a resolution to enduring problems such as the origin of the Born rule, the dissolution of the wave-function collapse problem and the selection of the interpretation of quantum mechanics. But how do we get there, and what is the starting idea? At the highest level of abstraction, the observer is understood by its participation in nature as the experimental testing of predictive theories, followed by falsification and refinements where appropriate. But as it stands these methods are informal, leading to ambiguities in their formulation. To eliminate these ambiguities, we will define experiments as reproducible protocols, and express them in terms of Turing complete languages and halting programs. A listing of such experiments is recursively enumerable and as an incremental contribution to knowledge serves as a foundation for the formalization of all falsifiable sciences. In turn this formalization leads to the remarkable result that the observer takes centre-stage as a measure space on all possible experiments, from which fundamental physics is entailed self-reflectively as the rules that observer participation in nature can and cannot do.