# NoR: 1.3 Rationalization. Structural vs. Substantive.

This is one of a series of posts setting out my work on the Nature of Representation. You can view the whole series by following this link

Radical Interpretation tells us: the correct interpretation of x is one which best rationalizes x’s dispositions to act in the light of the courses of experience x undergoes. But what is it for an interpretation to rationalize such things? There are thinner and thicker ways of understanding “rationalization” of actions/experiences, and this generates two very different versions of radical interpretation.

The first option is to identify rationality with structural rationality. A structurally rational agent is one whose mental states are patterned in the right way. For example, a paradigm constraint of structural rationality is formal consistency of belief. One violates this if one simultaneously believes that grass is green and believes that it is not green. Another paradigm of structural rationality is means-end coherence between beliefs/desires/actions. One violates that if one desires above all to get smarties, believes the only way to get smarties is to open the cupboard, yet fails to open the cupboard.

Such rational constraints are “structural” in that they’re insensitive to the specific contents of attitudes involved. It’s not anything to do with grass or greenness that makes it problematic to believe grass is green and that grass is not green. Schematically, for any p, it’s problematic to believe that p and that not-p. Likewise for the means-end constraint. Schematically, for any proposition q and act A, it’s problematic to desire (above all) to bring about q, believe that the only way to bring about q is to A, and yet fail to A. Structural rational constraints are characteristically topic-neutral, relying only on very abstract and formal specifications of the relations between attitudes/acts/experiences, for example, their logical forms (e.g. that the content of one belief is the negation of another).

Structural rationality constraints lend themselves to formal modelling. For a concrete example this consider (a minimal version of) Bayesianism.  The Bayesian starts from a distinctive take on the space of abstract interpretations. The version I’ll consider takes interpretations to map person-stages to an assignment of degree of belief in each proposition (probability) and degree of desirability in each proposition (utility). My Bayesian’s proposal for structural rational constraints include:

• Rationality constraints on beliefs over time: that they are updated by conditionalization on content extracted from experience.
• Rationality constraints on beliefs at a time: that they are probabilistic (i.e. satisfy the axioms of probability theory).
• Rationality constraints on final/instrumental desires and beliefs: that they fit means-end constraints articulated by (Jeffrey-style) decision theory.
• Rationality constraints on choices: that the agent chooses to do the thing they most desire to do, among the things they think they’re able to do.

This gives us a candidate metaphysics of representation: structural radical interpretation (Bayes-style). An interpretation will be correct if and only if it comes closest to making the agent’s dispositions to act (at t) dispositions to select the option that maximizes expected desirability (at t), and to making the beliefs and desires attributed evolve under the impact of experiences in the way the Bayesian demands.

These Bayesian constraints do not rule out wild initial belief states. It is rational, so far as the above constraints go, to have high conditional confidence that the world will explode tomorrow, given the course of experience you have undergone to this point—and so rational, after undergoing that experience, to end up believing that the world will explode tomorrow. Nor do Bayesian constraints rule out wild final desires, e.g. basic desires for a saucer of mud or indifference to what happens to you on future Tuesdays. That is by design: the core Bayesian story as articulated above was developed as a theory of the formal patterns that a well-run mind should exhibit, not about the particular contents that we have most reason to believe or desire. It would be a surprise if one could rule out wild initial belief states or desires on the basis of formal features alone (perhaps some have aspired to this, but it would be headline news if they really succeeded). I will take Bayesian structural rationality to be in this respect representative of structural rationality accounts more generally.

Yet there’s something crazy about a basic desire for a saucer of mud, or future-Tuesday indifference, and about humdrum experience triggering paranoid beliefs—something deeper and more alien than what’s wrong with commonplace false beliefs or unwholesome desires. In addition to constraints of rationality based on purely formal patterns among our attitudes, perhaps there are rational constraints that are sensitive to the particular contents we think or want. These would be constraints of substantive rationality. (Just to note: it’s perfectly in order for someone who advocates the Bayesian story above as an account of structural rationality, to think that there are in addition substantive rationality constraints floating around).

This generates our second option. Substantive radical interpretation is a metaphysics of content on which the correct interpretation of an agent is the one that does the best job of making her substantively as well as structurally rational. This is what Lewis (1974, 1992) explicitly advocates, as recent authors such as Pautz, Weatherson and Schwartz have emphasized. Unfortunately, Lewis never told us much about what these constraints of substantive rationality were, beyond a few examples (similar to those I have just mentioned: the saucer of mud, future-Tuesday indifference). What metaphysics of representation we get out will depend on what account of substantive rationality we feed in.

The view I will be developing here is substantive radical interpretation. Some of the work to be done is in describing what substantive rationality constraints amount to, and using this to predict and explain results in the philosophy of mind. But before I get into this level of detail, the next couple of posts will review the reasons that structural rational interpretation (unsupplemented) is untenable, and locate substantive radical interpretation as a natural reaction to this problem.