That quote about conjecture reminds me of a big point from Zen and the Art of Motorcycle Maintenance. The author suggests that 'science' / 'the scientific method' don't actually account for the process by which ideas/hypotheses come into existence, science only comes into play once the hypothesis appears (from whence does it appear?). He calls that magic smoke 'Quality'. (Using the language you cited, I guess we would be asking about where the conjecture itself comes from). I'm realizing now that this is tangential to your point, sorry, but thanks for posting this interesting comment.

It's an interesting tangent! I agree with that idea, what we are taught as the "scientific method" does kind of assume you've already had the idea. The method helps you test or criticise it. I don't remember if Deutsch has an opinion on where conjecture comes from, but I think it's bound up with creativity... "quality" sounds like an interesting way of putting it.