Hard to Vary or Hardly Usable?
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With an account, you can revise, criticize, and comment on ideas.My critique of David Deutsch’s The Beginning of Infinity as a programmer. In short, his ‘hard to vary’ criterion at the core of his epistemology is fatally underspecified and impossible to apply.
Deutsch says that one should adopt explanations based on how hard they are to change without impacting their ability to explain what they claim to explain. The hardest-to-change explanation is the best and should be adopted. But he doesn’t say how to figure out which is hardest to change.
A decision-making method is a computational task. He says you haven’t understood a computational task if you can’t program it. He can’t program the steps for finding out how ‘hard to vary’ an explanation is, if only because those steps are underspecified. There are too many open questions.
So by his own yardstick, he hasn’t understood his epistemology.
You will find that and many more criticisms here: https://blog.dennishackethal.com/posts/hard-to-vary-or-hardly-usable
I’ve just posted the following update on my article linked in the parent idea. The bounty can be closed, at least for now.
Update 2026-09-06
TL;DR: Computational underspecification may have been solved; HTV is still not a description of rationality or of the cause of progress.
Dirk Meulenbelt, Benjamin Davies, and I ran a Veritula bounty worth USD 3,600 to rehabilitate HTV with regard to its computational underspecification. Jad Elmourad came up with a simple idea:
- Prompt the user for two explanations.
- Prompt the user for variants of each.
- Compare the count of variants and rank accordingly (lower count = better).
Unfortunately, the integrity of the discussion of this idea can’t be warranted because of problematic participation in the bounty. As a result, it’s currently not known whether the idea has pending criticisms, ie, whether its advocacy is rational.
That said, the program runs. I’ve tried it out. It offers a simple alternative to the sliders above. It does seem to partially rehabilitate HTV specifically with regard to its previous computational underspecification. In other words, where proponents of HTV used to be unable to explain exactly how to determine how HTV an explanation is, they can now follow the steps above and answer accordingly. They can now rank explanations this way. That helps.
What the program also shows, maybe ironically, is that nobody actually follows those steps in real life. For example, scientists don’t count variants of theories like that. Deutsch is still right about the following: “Whenever a wide range of variant theories can account equally well for the phenomenon they are trying to explain, there is no reason to prefer one of them over the others, so advocating a particular one in preference to the others is irrational.” (BoI chapter 1; also, the ‘grass cure’ from FoR comes to mind as an example.) So when a theory is easy to vary in this sense, that does indeed mean we can’t form a rational preference for one of the variants, at least not without additional information. But that doesn’t mean all preference formation involves theories being hard to vary.
What scientists actually do is criticize a theory for being ‘easy to vary’ (they’d just call it ‘arbitrary’), reject it, and look for a different one. They wouldn’t care to rank theories this way. And they’d use HTV as only one mode of criticism among many. It’s not the core of rationality and progress; again, not all problems of preference formation can be reduced to having too many variants.
Philosophers don’t rank theories this way either. I don’t recall a single instance in BoI where Deutsch counts variants and justifies a preference that way. Instead, he lists and addresses criticisms. He follows the Veritula method.
Also, one would only enter variants into the program that have already survived criticism. So then the question is how scientists tell whether an idea has survived criticism. And there the answer is still the Veritula method. So the program could maybe be incorporated into Veritula as an additional step. But again, it’s not needed if ETV is simply one kind of criticism among many, with no ranking.
To be clear, that isn’t a criticism of the program – the bounty specifically asked for the ranking. Here, I’m only exploring the wider implications if the program can be advocated rationally. And the answer is: the computational underspecification is then solved, but HTV is still not a description of rationality or of the cause of progress. That idea, as I just phrased it, can be adopted rationally at the time of writing, and we can move forward accordingly.