I'm on the fence between you two. On one hand, it's true that missing pieces in a mathematical symmetry often motivate us to go find the real-world thing we hav

I'm on the fence between you two. On one hand, it's true that missing pieces in a mathematical symmetry often motivate us to go find the real-world thing we haven't observed. On the other hand, your example of a multi-layered substance that exhibits negative refraction does seem like "cheating". We can make transistors by layering different doped silicone variants together, but it's not considered a fundamental electrical component. If memristors require the same amount of engineering, are they "natural"? What would be fascinating is, what if the missing puzzle piece implies an exotic shaping of "just" wire that would act as a memristor? Some crazy contortion like the magnetic field designs I've seen for containing fusion reactions.