Hard to Vary or Hardly Usable?
Showing only those parts of the discussion that lead to #3838.
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With an account, you can revise, criticize, and comment on ideas.Deutsch’s stance in my own words:
The distinguishing characteristic between rationality and irrationality is that rationality is the search for good explanations. All progress comes from the search for good explanations. So the distinction between good vs bad explanations is epistemologically fundamental.
A good explanation is hard to vary “while still accounting for what it purports to account for.” (BoI chapter 1 glossary.) A bad explanation is easy to vary.
For example, the Persephone myth as an explanation of the seasons is easy to change without impacting its ability to explain the seasons. You could arbitrarily replace Persephone and other characters and the explanation would still ‘work’. The axis-tilt explanation of the earth, on the other hand, is hard to change without breaking it. You can’t just replace the axis with something else, say.
The quality of a theory is a matter of degrees. The harder it is to change a theory, the better that theory is. When deciding which explanation to adopt, we should “choose between [explanations] according to how good they are…: how hard to vary.” (BoI chapter 9; see similar remark in chapter 8.)
From my article:
Isn’t the assignment of positive scores, of positive reasons to prefer one theory over another, a kind of justificationism? Deutsch criticizes justificationism throughout The Beginning of Infinity, but isn’t an endorsement of a theory as ‘good’ a kind of justification?
The criterion for HTV applied to 2 explanation is not justificationism I think. It allows to say explanation A is better than explanation B, which is equivalent to: explanation B is worse than explanation A. So it is a relative claim about an explanation, relative to another, not versus some absolute criterion of goodness. Similar to a crucial test (e.g. Eddington): we refute Newton's theory and keep Einsteins, that is not a claim about the goodness of Einsteins theory, that theory merely has survived, it has not gotten "goodness points". It could be refuted always later on by any better theory, in which case we would drop it too.
So it is a relative claim about an explanation, relative to another, not versus some absolute criterion of goodness.
So what? I didn’t mention an absolute criterion. My original criticism already applies to both relative and absolute criteria of quality (what you call “goodness”).