Hard to Vary or Hardly Usable?
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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?
@lola-trimble suggested during a space that a theory is hard to vary if it’s not easy to vary. So the maximum score would be 0, not +1,000 or whatever. In which case ‘hard to vary’ isn’t an endorsement.
Even so, if a criticism gets score -10, that will push the parent theory’s score above 0.
But then the ease with which a criticism could be varied might have no effect on its parent. So why even bother having a notion of ‘easiness to vary’ at that point?
But calling a theory ‘good’ sounds like an endorsement. Deutsch also writes (BoI chapter 10) that a “superb” theory is “exceedingly hard to vary”. Ultimately we’d have to ask him, but for now, given the strength and positivity of those terms, I think it’s fair to conclude that he means ‘hard to vary’ as an endorsement.
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.
Similar to a crucial test …
But that’s exactly where HTV differs from Popper. Popper doesn’t give a theory points when it survives a crucial test. HTV does. From BoI chapter 1:
… testable explanations that have passed stringent tests become extremely good explanations …
That's because a good explanation for Deutsch is not an explanation with good points, but an explanation that is harder to vary compared to any other explanation. So again relative to other explanations.
The word "good" is indeed misleading in that sense, but he clearly qualifies it as performing better, relative to other explanations, on his HTV criterion, and not as: the explanation having scored high points.
with good points
I didn’t say the explanation doesn’t make good points, I said the explanation doesn’t get points.
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”).
Because relative criteria are fine to posit and not justificationist. We can propose criteria that claim that explanation A is better than explanation B without that being justificationism
From BoI chapter 1 glossary:
The misconception that knowledge can be genuine or reliable only if it is justified by some source or criterion.
That says nothing about absolute vs relative. Stop making up stuff.
@liberty-fitz-claridge says (#3885) it’d be implausible for HTV to be justificationist since that would contradict the rest of Deutsch’s anti-justificationist philosophy.
I don’t think that alone means my interpretation of HTV is implausible. We’re bound to find contradictions eventually. In a good book like BoI, they’re just rare, so when we do find them, they go against the bulk of the philosophy.