I'm sure this talk will be of interest, even if most of the ideas that he talks about will be familiar to readers here.
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In this talk David Deutsch discusses "the most important discovery in human history"; how humanity moved beyond a few hundred thousand years of complete ignorance about the universe. Deutsch attempts to be specific about what led to this change - he concludes that it is the insistence that an explanation be 'hard to vary'.
Whilst a 'hard to vary' explanation is functionally the same as a, more commonly known, Occam's Razor explanation (since fewer parameters necessarily make a fit harder to vary) the slightly different emphasis might be a useful pedagogical tool. A 'hard to vary' explanation will perhaps lead more naturally to questions about strong predictions and falsifiability than Occam's razor. It also seems harder to misunderstand. As we know, Occam's razor suffers because of the difference between actual complexity and linguistic complexity, so an explanation like "it's magic" can appear to be simple. Magic might appear simple, but it will never appear 'hard to vary', so students of rationality would have one less pitfall awaiting them.
Deutsch also touches on what constitutes understanding and knowledge and cautions us not to trust predictions that are purely of an extrapolated empirical nature as there is no true understanding contained there.
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If you haven't already read Deutsch's book "The Fabric of Reality" I'd highly recommend that as well.
He does not mean "lacking unnecessary details". For example the statements "Everyone just acts in his own interest" or "Everyone is really an altruist" are simple and lack unnecessary details, explain quite a lot, and are consistent with Occam's razor. But by Deutsch's criteria they are bad explanation because they are too easy to vary. For example, someone who believes in the self-interest theory could say, "John gave to charity because he would have felt guilty otherwise. So he really was selfish" .
We see that it is easy to change the theory that everyone is selfish to accomodate the case of someone who seems altruistic.
Or someone who believes in the altruist theory could say about John murdering Harry, "Well then, Harry must have been very unhappy."
The altruist theory and the selfishness theory are simple and explanatory in their own way, but too easy to vary. Similarly the idea that that sexism, feminism, capitalism, communism, parental coercion, environmental disregard, etc. cause unhappiness or mental illness or some other broad conclusion are equally meaningless. These explanations are bad because they can be varied to explain ANYTHING.
In contrast, theories that are difficult to vary go out on a limb. They are bold conjectures that explain a lot but even one small counterexample easily invalidates the whole thing. A good theory can not easily be changed to "take into account" the aberration. For example, Einstein's theory of gravitation is a good explanation because it explains a lot, it makes counterintuitive predictions, and even one repeatable counterexample invalidates the whole thing. It can't be easily changed to accommodate something else without invalidating everything else about it.
Theories that are hard to vary remain constant over time. They are more true and therefore more timeless. Invariable theories possess more verisimilitude ("truth-likeness" to use Popper's term).
Like the very best possible theory, truth also cannot be varied. It is completely timeless. It was, is, and always will be true, without any change. That is Deutsch's point.