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Reflective equilibrium between physics and philosophy, and between GR andparticle physics, is fruitful and rational. I consider the virtues ofsimplicity, conservatism, and conceptual coherence, along with perturbativeexpansions. There are too many theories to consider. Simplicity suppliesinitial guidance, after which evidence increasingly dominates. One should startwith scalar gravity; evidence required spin 2. Good beliefs are scarce, sodon't change without reason. But does conservatism prevent conceptualinnovation? No: considering all serious possibilities (Feynman, Weinberg, etc.)could lead to Einstein's equations. (The rehabilitation of massive gravityshows that 'progress' isn't unidirectional.) GR is surprisingly intelligible.Energy localization makes sense if one believes Noether mathematics: aninfinity of symmetries shouldn't produce just one energy. Hamiltonian changeresults from Lagrangian-equivalence. Causality poses conceptual questions. ForGR, what are canonical 'equal-time' commutators? For massive spin 2, backgroundcausality exists but is violated. Both might be cured by engineering abackground null cone respected by a gauge groupoid. Perturbative expansions canenlighten. They diagnose Einstein's 1917 'mass'-Lambda analogy.Ogievetsky-Polubarinov (1965) invented an infinity of massive spin 2 gravities-- including ghost-free de Rham-Gabadadze-Tolley (2010) theories! --perturbatively, and achieved the impossible (c.f. Weyl, Cartan): spinors incoordinates.

Progress and Gravity: Overcoming Divisions between General Relativity and Particle Physics and between Physics and HPS