Existence and Consciousness as One: How Sufficient Reason Grounds Both

We develop a structural argument from a structural reading of the Principle of Sufficient Reason (PSR) for the co-constitution of existence and consciousness at cosmic scale — a position we call necessary cosmopsychism. The argument proceeds conditionally: if the universe is explainable, what structural features must it have? The explainability condition rules out all linear causal structures — external first cause, infinite regress, brute first fact — and entails unidirectional causation, requiring a closed causal topology. This loop produces a selection problem for fixed-point initial conditions, which are structurally distinguished by satisfying a unique self-consistency condition that no internal mechanism can ground. We argue the problem dissolves when the complete structure has a coextensive cognitive system, and a direct argument identifies this system with phenomenal consciousness at cosmic scale: a cognitive system coextensive with the universe is interior to everything, and that interiority-to-everything just is phenomenal consciousness. The argument's deepest claim is that existence and consciousness are co-constitutive at cosmic scale: existence under structural PSR requires cosmic consciousness — the structural derivation — and cosmic consciousness requires existence — consciousness needs a bearer. The argument arrives at cosmopsychism via the explainability question rather than via the hard problem directly, engages the combination, decombination, zombie, and circularity objections, and arrives at a position convergent with Russellian monism as a consequence of the argument rather than as a starting premise.