Defining the Observer

Defining the Observer: A General Physical Criterion, Its Relation to Existing Models, Impossibility of the final ascent, and the Impossibility of the Ideal Limit. No treatment of quantum gravity that takes observers seriously can avoid the question of what an observer physically is, and existing treatments - including the author's own (Materov 2026a,b,c) - each state a modelling assumption for the observer without deriving it. This paper's contribution is narrow and specific, not a claim to be the first physical account of an observer: a particular combination of requirements (D1–D4) on the trajectory of an embedded tensor factor, a separately-argued self-capacity criterion for individuation (D3′) tied to the no-cloning theorem, and a short, self-contained no-go argument (Proposition 4.1) for complete reconstruction of an observer's complement. We show that the observer-notions already in use in Materov (2026a,b,c) relate to this combination in three structurally different ways - as a more general object, as a concrete realization, and as a structurally parallel system not yet unified with it - and work out one concrete case, the photonic Friend of Local-Friendliness experiments, in detail. Since no observer can be ideal in this omniscient sense, we instead investigate uniqueness for a weaker, separately-defined notion (a self-dominant observer, whose own representational capacity exceeds its observing capacity); no dimension-counting argument we have found forces this either, and the question remains, honestly, open.