It is a fact about our social and epistemic lives that Rationality is a norm that guides our thinking about beliefs and hypotheses; it is a Concept we use to direct thinking and discourse. (Habermas would call it a „regulative ideal“.) Any plausible conception of rationality has to make sense of this basic fact.
This also means that it has to show that we are mostly rational beings: The norm would be useless if the course of action it demands wasn’t viable—if we had to change large parts of our belief network to fulfil its demands. As Donald Davidson puts it,
the possibility of irrationality depends on a large degree of rationality. Irrationality is not mere lack of reason but a disease or perturbation of reason.Davidson (1982), 99
If we want to explain empirically why rationality is a social and epistemic norm, we can call upon Evolution:
As biological creatures, rationality points us to Models of the world and to courses of action that increase our chance of success in our natural Environment. Its basic principles have evolved through variation and Selection into what they are exactly because of their contribution to this success.
In culture and society, our idea of rationality evolves according to its contribution to success in other contexts: We optimise for winning at the table and for discovery in science, for conflict resolution and for short, but affordable flights.
So in both the biological and the social context, as a social and epistemic norm, rationality is an evolved Strategy – just as all Concepts evolve to be useful. In its “unarticulated” use, it is part and parcel of our (more or less) successful dealing with an evolutionary environment. In its “articulated” use, Rationality means reaching a reflective equilibrium.
References
- Davidson (1982): “Rational Animals”, in: Subjective, Intersubjective, Objective