Chapter 5 considers the claim that, as what the normative principles of rationality are and what our reasoning competence is are both determined by a process of reflective equilibrium, the two cannot diverge. This claim, if true, would present an a priori reason for interpreting any divergence from the norms, such as the results of the reasoning experiments, as performance errors. But Stein argues that this is not the case. Empirical evidence will play a role in the latter process which, at least according to SPR, it will not in the former. Chapter 6 concerns not a conceptual argument for RT, but an empirical one.
Biological evolution is caused by natural selection.
Natural selection favours optimal traits. The optimal cognitive mechanisms are truth-tropic. Therefore, biological evolution favours cognitive mechanisms that are truth-tropic. A truth-tropic cognitive mechanism is a rational one. Therefore, biological evolution favours rational cognitive mechanisms.