Simple Refutation of David Deutsch’s ‘Hard to Vary’
David Deutsch says we form rational preferences by choosing explanations that are hardest to vary. But that doesn’t work. I explain why.
Epistemologists work on ways to form a rational preference for one idea over another.
In The Beginning of Infinity, David Deutsch suggests a way based on “good explanations”. From the chapter 1 glossary (31):
Good/bad explanation An explanation that is hard/easy to vary while still accounting for what it purports to account for.
If you can easily change an explanation without breaking its ability to explain whatever it claims to explain, it’s easy to vary, meaning bad. If you can’t, it’s hard to vary, meaning good. The harder, the better.
He says to prefer the idea that’s hardest to vary.
The specific method comes later in the book, outside the main discussion of hard to vary in chapter 1. He only mentions it twice in passing, in other contexts (math and voting, not epistemology generally). As a result, many readers miss it. Chapter 8 (191):
… the overall method, as in all fields, is to make conjectures and to criticize them according to how good they are as explanations.
Chapter 9 (209):
… we should choose between [scientific theories] not on the basis of their origin, but according to how good they are as explanations: how hard to vary.
In chapter 7, Deutsch suggests a yardstick for having understood a computational task: “if you can’t program it, you haven’t understood it.” (154)
Forming a rational preference for one idea over another is a computational task. You could in principle write down step-by-step instructions to complete this task, in the form of an algorithm.
Since nobody has programmed a method to form a rational preference using hard to vary, nobody has understood it.
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