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Consistency and rationality

from Lars Syll

consistentAxioms of ‘internal consistency’ of choice, such as the weak and the strong axioms of revealed preference … are often used in decision theory, micro-economics, game theory, social choice theory, and in related disciplines …

Can a set of choices really be seen as consistent or inconsistent on purely internal grounds, without bringing in something external to choice, such as the underlying objectives or values that are pursued or acknowledged by choice? …

The presumption of inconsistency may be easily disputed, depending on the context, if we know a bit more about what the person is trying to do. Suppose the person faces a choice at a dinner table between having the last remaining apple in the fruit basket (y) and having nothing instead (x), forgoing the nice-looking apple. She decides to behave decently and picks nothing (x), rather than the one apple (y). If, instead, the basket had contained two apples, and she had encountered the choice between having nothing (x), having one nice apple (y) and having another nice one (z), she could reasonably enough choose one (y), without violating any rule of good behavior. The presence of another apple (z) makes one of the two apples decently choosable, but this combination of choices would violate the standard consistency conditions, including Property a, even though there is nothing particularly “inconsistent” in this pair of choices (given her values and scruples) … We cannot determine whether the person is failing in any way without knowing what he is trying to do, that is, without knowing something external to the choice itself.

Amartya Sen

Being able to model a credible world, a world that somehow could be considered somehow ‘similar’ to the real world is not the same as investigating the real world. The minimalist demand on models in terms of ‘credibility’ and ‘consistency’ has to give away to stronger epistemic demands. Claims in a ‘consistent’ model do not per se give a warrant for exporting the claims to real-world target systems.

Questions of external validity are important more specifically also when it comes to microfounded macro models. It can never be enough that these models somehow are regarded as internally consistent. One always also has to pose questions of consistency with the data. Internal consistency without external validity is worth nothing.

Yours truly has for many years been urging economists to pay attention to the ontological foundations of their assumptions and models. Sad to say, economists have not paid much attention — and so modern economics has become increasingly irrelevant to the understanding of the real world.

As long as mainstream economists do not come up with any export-licenses for their theories and models to the real world in which we live, they really should not be surprised if people say that this is not science.

To have ‘consistent’ models and ‘valid’ evidence is not enough. What economics needs are real-world relevant models and sound evidence. Aiming only for ‘consistency’ and ‘validity’ is setting the economics aspirations level too low for developing a realist and relevant science.

Economics is not mathematics or logic. It’s about society. The real world.

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