Abstract Biology is distinguished from chemistry not by different matter but by a different mode of constraint. Where molecular structure is constraint stabilised under bare reflexivity, biological organisation is constraint maintained under integrated reflexivity: the organism registers its own boundary conditions and steers participation to preserve them. This paper develops a systematic analysis of biological identity within the Triaxial Existential Field (TEF) programme. Function, traditionally understood as irreducible teleology, is retyped as cost-bearing reflexive constraint: the heart is "for" pumping not because purpose is primitive but because the organism is a reflexively integrated system whose closure depends on circulatory constraint, and whose failure imposes irrecoverable cost. Life is not a substance or vital force but integrated C–R–P closure at organism scale — a regime where coherence, reflexivity, and participation form a self-maintaining loop. The account is substrate-neutral: what matters is the structural organisation, not the material substrate. The paper engages the etiological tradition in philosophy of biology (Millikan, Godfrey-Smith, Neander), organisational closure frameworks (Maturana and Varela, Moreno and Mossio), eliminativist programmes (Rosenberg), and the Kantian antinomy of teleological judgement. It argues that the structural account is constitutive of function, of which the etiological account is a historical correlate; that the register distinction is structurally necessary for any non-eliminative, non-mysterian biology; and that the framework generates a concrete verdict divergence from organisational closure accounts — a synthetic protocell satisfying closure of constraints but lacking R-integration is classified as alive by Moreno and Mossio but not by TEF, and real biological borderline cases (viruses, prions, minimal cells) receive clean, structurally grounded verdicts under the tri-criterion test. The biogenesis threshold is formalised through a cost crossover inequality: the entropic cost of passive persistence scales exponentially with non-redundant complexity while the metabolic cost of reflexive maintenance scales subexponentially, yielding a critical complexity above which closure is the only cost-efficient mode of robust persistence. This transforms the origin of life from a contingent accident into a structurally expected phase transition. Two implications are developed in detail: cancer as constraint conflict between nested closures, and homochirality maintenance as cost-bearing reflexive constraint — redeeming the chemistry paper's explicit promise to derive the biological mechanisms that keep enantiomeric excess below the admissibility threshold. Falsification conditions are specified, including an interventional test for function without reflexive integration, a minimum information-processing prediction, and five conditions under which the biogenesis threshold argument fails.