 The conventional elements in physical theories were often misunderstood by Poincaré’s contemporaries. So, for example, his affirmation that neither of the two propositions ‘The earth turns round’ and ‘The earth does not turn round’ is “more true than the other in the kinematic sense,” is not a rehabilitation of Ptolemy’s system, but the consequence of the fact that in physics, the empirical systematic (not epistemic) under-determination of theories is limited by unifying considerations: “A physical theory is by so much the more true, as it puts in evidence more true relations.” So, the ‘The earth turns round’ has a richer content expressed by the “flattening of the earth, the rotation of Foucault’s pendulum, the gyration of cyclones, the trade-winds, and who knows what else?” (Poincaré 1905b: 184–85; 1913b: 353). However, in physics proper—that is, in optics and in electrodynamics—the conventional elements seem to be further weakened: