A behaviour and by extension a Behavioural Disposition is instrumentally rational iff it maximises the subjective probability for a System to achieve a certain goal given an Environment and the available Knowledge about it.
Someone displays instrumental rationality insofar as she adopts suitable means to her ends.Stanford Encyclopedia of Philosophy
Such an understanding of rationality
is within the intersection of all theories of rationality (and perhaps nothing else is). In this sense, instrumental rationality is the default theory, the theory that all discussants of rationality can take for granted, whatever else they think.Nozick (1993), 133
On this view, Rationality is an evolved strategy, aiming for success within an evolutionary environment.
Beyond that, Rationality means reaching a reflective equilibrium, which is also true of a Theory of rationality itself – it has to reach an equilibrium between its empirical and normative parts. The latter are a reflection of the fact that A theory of rationality articulates practices.
Perceived irrationality is rooted in actual rationality in various non-obvious ways.
References
- Nozick (1993): The Nature of Rationality
- Stanford Encyclopedia of Philosophy, “Instrumental Rationality”