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I think that's a misreading. If 'hard to vary' is a fixed criterion used to measure the value of an explanation, it would go against Deutsch's own criticisms of justificationism (various chapters of BoI and FoR) as well as his criticisms of scientism – that is, the misapplication of scientific methods to philosophical problems (e.g. https://www.youtube.com/watch?app=desktop&v=tzWGfi4XhLA&t=7s). I see why you would interpret the BoI quote in that way, but in the context of the whole philosophy your interpretation is implausible.

Regardless of what Deutsch meant, though, the main point is that it's possible to talk about the virtues of explanations without falling into justificationism, for example when trying to explain progress.

I have made related points in #3883.

I see why you would interpret the BoI quote in that way, but in the context of the whole philosophy your interpretation is implausible. It would go against Deutsch's own criticisms of justificationism (various chapters of BoI and FoR) as well as his criticisms of scientism – that is, the misapplication of scientific methods to philosophical problems (e.g. https://www.youtube.com/watch?app=desktop&v=tzWGfi4XhLA&t=7s).

Regardless of what Deutsch meant, though, the main point is that it's possible to talk about the virtues of explanations without falling into justificationism, for example when trying to explain progress.

I have made related points in #3883.

It seems that you've taken the idea of hard to vary as saying that the process of choosing between competing theories is just about measuring how much of this trait they have. One clearly wouldn’t get better explanations from doing that, as it would just be a mechanical way of judging theories.

But this is a misunderstanding of hard to vary, which is simply an outcome of the process of conjecture and criticism. It's not a criterion to be applied before any critical discussion has taken place.

The same goes for other aspects of good explanations, such as depth, accuracy, elegance, reach. These are things we aspire to have, but they are not criteria for judging theories, for which we need no justification.