I don't understand why this is a valid argument against physicalism. Maybe I can't get it because the definition of "physicalism" seems muddy and unintuitive to

I don't understand why this is a valid argument against physicalism. Maybe I can't get it because the definition of "physicalism" seems muddy and unintuitive to me. Or maybe because there's an actual difference between knowing how something will work (light -> retina -> neurons etc.) and experiencing it while it works. But all of this is physical: both the knowledge, as well as the memory of experience. That's the problem with many philosophers: they create some concepts, define them in arbitral (not necessarily adequate, often misleading) way and then other philosophers discuss with these concepts. IMHO most of the time it doesn't bring useful conclusions and doesn't advance human knowledge.