In my Thoughts, in section 23, I explained why Feynman called the double-slit experiment the only mystery of quantum mechanics. The idea is that Feynman called the wave-particle duality a mystery because he was confused by language. Language believes in “things”, but strictly speaking there are no “things”, and therefore there are no particles. There is only flux, and language falsifies the reality of flux. So, when he observed that a so-called particle could behave like a wave under certain conditions, he called it a mystery. But he was just confused by the inadequacy of language to properly describe reality. There’s really no mystery, then, that’s just how stuff works at the minuscule level of quantum mechanics, and language makes it look paradoxical because it believes in “things” and therefore particles.