If humans are bad at mental arithmetic, but good at, say, not dying - doesn't that suggest that, as a practical matter, humans should try to rephrase mathematical questions into questions about danger?

E.g. Imagine stepping into a field crisscrossed by dangerous laser beams in a prime-numbers manner to get something valuable. I think someone who had a realistic fear of the laser beams, and a realistic understanding of the benefit of that valuable thing would slow down and/or stop stepping out into suspicious spots.

Quantifying is ONE technique, and it's been used very effectively in recent centuries - but those successes were inside a laboratory / factory / automation structure, not in an individual-rationality context.

Steve Keen's Debunking Economics blames debt, not automation.

Essentially, many people currently feel that they are deep in debt, and work to get out of debt. Keen has a ODE model of the macroeconomy that shows various behaviors, including debt-driven crashes.

Felix Martin's Money goes further and argues that strong anti-inflation stances by central bank regulators strengthen the hold of creditors over debtors, which has made these recent crashes bigger and more painful.

The two comments, though contradictory, refer to two different thought experiments.

Is it reasonable to take this as evidence that we shouldn't use expected utility computations, or not only expected utility computations, to guide our decisions?

If I understand the context, the reason we believed an entity, either a human or an AI, ought to use expected utility as a practical decision making strategy, is because it would yield good results (a simple, general architecture for decision making). If there are fully general attacks (muggings) on all entities that use expected utility as a practical decision making strategy, then perhaps we should revise the original hypothesis.

Utility as a theoretical construct is charming, but it does have to pay its way, just like anything else.

P.S. I think the reasoning from "bounded rationality exists" to "non-Bayesian mind changes exist" is good stuff. Perhaps we could call this "on seeing this, I become willing to revise my model" phenomenon something like "surprise", and distinguish it from merely new information.

Magic Haskeller and Augustsson's Djinn are provers (or to say it another way, comprehensible as provers, or to say it another way, isomorphic to provers). They attempt to prove the proposition, and if they succeed they output the term corresponding (via the Curry-Howard Isomorphism) to the proof.

I believe they cannot output a term t :: a->b because there is no such term, because 'anything implies anything else' is false.

The type constructors that you're thinking of are Arrow and Int. Forall is another type constructor, for constructing generic polymorphic types. Some types such as "Forall A, Forall B, A -> B" are uninhabited. You cannot produce an output of type B in a generic way, even if you are given access to an element of type A.

The type corresponding to a proposition like "all computable functions from the reals to the reals are continuous" looks like a function type consuming some representation of "a computable function" and producing some representation of "that function is continuous". To represent that, you probably need dependent types - this would be a type constructor that takes an element, not a type, as a parameter. Because not all functions are continuous, the codomain representing "that function is continuous" isn't in general inhabited. So building an element of that codomain is not necessarily trivial - and the process of doing so amounts to proving the original proposition.

What I'm trying to say is that the types that type theorists use look a lot more like propositions than the types that mundane programmers use.

I think you may be sincerely confused. Would you please reword your question?

If your question is whether someone (either me or the OP) has committed a multiplication error - yes, it's entirely possible, but multiplication is not the point - the point is anthropic reasoning and whether "I am a Bolzmann brain" is a simple hypothesis.

The arithmetical hierarchy is presuming a background of predicate logic; I was not presuming that. Yes, the type theory that I was gesturing towards would have some similarity to the arithmetical hierarchy.

I was trying to suggest that the answer to "what is a prediction" might look like a type theory of different variants of a prediction. Perhaps a linear hierarchy like the arithmetical hierarchy, yes, perhaps something more complicated. There could be a single starting type "concrete prediction" and a type constructor that, given source type and a destination type, gives the type of statements defining functions that take arguments of the source type and give arguments of the destination type.

The intuitive bridge for me from ultrafinitism to the question "what counts as a prediction?" is that ultrafinitists do happily work with entities like 2^1000 considered as a structure like ("^" "2" "1000"), even if they deny that those structures refer to things that are definitely numbers (of course, they can agree that they are definitely "numbers", given an appropriately finitist definition of "number"). Maybe extreme teetotalers regarding what counts as a prediction would happily work with things such as computable functions returning predictions, even if they don't consider them predictions.

Perhaps there is a type theory for predictions, with concrete predictions like "The bus will come at 3 o'clock", and functions that output concrete predictions like "Every monday, wednesday and friday, the bus will come at 3 o'clock" (consider the statement as a function taking a time and returning a concrete prediction yes or no).

An ultrafinitist would probably not argue with the existence of such a function, even though to someone other than an ultrafinitist, the function looks like it is quantifying over all time. From the ultrafinitist's point of view, you're going to apply it to some concrete time, and at that point it's going to output a some concrete prediction.