How much rationality? 1. Lab 2. Rationality / Reasonableness / Phronēsis (Practical Wisdom) 3. Approaches to reasonableness
Integrated interactive explainer

How much rationality?

Core claim: formal rationality is not the enemy. The error is to mistake it for a complete theory of judgment. As models become incomplete, uncertainty rises, or ruin enters the possibility space, more of the burden shifts toward reasonableness, practical wisdom, and robustness.

I. Decision Lab: when does calculation stop being enough?

Move the three variables. The page estimates how much judgment the situation requires. This is not an empirical model. It is a conceptual instrument for seeing why “more calculation” is not always the same as “better reasoning.”

Model completeness

How much of the relevant state space and causal structure is actually represented?

80

Uncertainty

How unstable, noisy, or genuinely unknowable are the relevant conditions?

30

Stakes

How costly is a locally sensible but globally mistaken action?

50

Judgment load: /100

Model incompleteness → Uncertainty → reasonableness Phronēsis (Practical Wisdom)/ robustness formal rationality reasonableness Stakes are not plotted; sufficiently high ruin-risk can shift the recommendation toward robustness.

Talebian amendment

When ruin is in the possibility space, expected-value optimization can become the wrong objective. Survival, optionality, reversibility, and margin of safety become epistemic virtues.

Toulminian amendment

Even absent ruin, case-sensitive judgment becomes necessary once formal criteria leave too much out. The issue is not anti-rationality, but the limits of rationality as a total method.

II. Three modes of judgment

Once the lab clarifies the environment, the next question is: what kind of judgment does the case call for? These three modes are not simple rivals. Often the later modes contain the earlier as subordinate instruments.

Rationality

Make the rule explicit

Prefer formal criteria, consistency, universality, and transparent derivation.

Best when the state space is stable and the objective is clear.

Reasonableness

Fit the rule to the case

Prefer proportion, context, exceptions, and practical adequacy.

Best when formal rules underdescribe the situation.

Phronēsis (Practical Wisdom)

Judge what matters here

Integrate ends, salience, timing, character, and situational particulars.

Best when even the right objective is partly at issue.

Test a case

Tax calculation

Relative fitness of the selected mode

Rationality Reasonableness Phronēsis(Practical Wisdom) rules • consistency optimization • proof context • exceptions proportion • adequacy ends • salience • timing character • judgment As model incompleteness rises, judgment must absorb more of what the model omits.
Architectural point: the practical contrast is usually not “rationality or reasonableness?” but rather “how much of the task can formal rationality carry before residual judgment takes over?”

III. Approaches to reasonableness

Choose the failure mode

Toulmin

Move: fit reasons to the case rather than force the case into universal rules.

Context, warrants, exceptions, practical adequacy.

Dewey

Move: test ideas by using them to resolve concrete problematic situations.

Inquiry, experiment, consequences, reconstruction.

Rorty

Move: abandon the hope for a final vocabulary.

Recontextualization, contingency, anti-foundationalism.

Joas

Move: treat action as creative and ends as emergent.

Embodiment, situation, value formation, non-teleological action.

Chapman

Move: replace totalizing systems with context-sensitive participation.

Nebulosity, pattern, participation, anti-eternalism.

Taleb

Move: treat hidden tail risk and model error as central.

Robustness, optionality, asymmetry, anti-fragility, ruin-avoidance.

ThinkerPrimary enemyUnit of correctionWhat replaces certainty?
ToulminFormalism detached from casesArgument / judgmentReasonableness
DeweySpectator theory of knowledgeInquiry / situationExperimental warranted assertibility
RortyRepresentational foundationalismVocabulary / conversationRecontextualization and solidarity
JoasTeleological action theoryEmbodied actionCreative adjustment
ChapmanEternalist system-buildingParticipation / patternFlexible engagement with nebulosity
TalebFragile optimization under hidden tail riskExposure / system design / decision postureRobustness, optionality, ruin-avoidance
Six correctives to overformal rationality Good judgment does not begin by assuming that explicit formal models capture everything that matters. Toulminfit reasons to the case Deweytest ideas in the situation Rortyrecontextualize Joasact creatively Chapmanparticipate flexibly Talebdesign for survival

One wrong question

“Which is superior, rationality or reasonableness?” is confused. Formal rationality is often indispensable. The live question is how far it carries before the remainder must be absorbed by contextual judgment, practical wisdom, or robustness.