Let us imagine that someone claims to be able to explain everything in terms of fundamental physics, or claims that ultimately everything is explicable in such terms. They’re almost certainly wrong, but let’s just go with it for now.
Note that one thing they will not have explained is fundamental physics itself. This may seem obvious and not even worth stating, but a lot of the task of a philosopher is IMO to state the obvious and to do so clearly. Let me quote from philosopher Brian Earp, from a discussion of consciousness: “Positing, ordinarily understood, is a very different enterprise from explaining. And it is typically much easier to do. To explain something (typically) just is to give a reductive or mechanistic account of it. Or, if that cannot be done, at minimum, it is to show why something exists, or is the way it is, by referring to at least one other thing and tracing some sort of entailment A to B. To posit, by contrast, requires much less work. You need only to declare, “The thing exists because it just does” – a statement which might even be true, but which does not give the sense of ah-hah! that is typically associated with genuine explanation”
So, an interesting question arising from this is “what is the best fundamental level of explanation?” It is not at all clear to me that fundamental physics is that level (i.e. I’m pretty sure it isn’t), for at least two reasons. Firstly, it seems to me to cry out for further explanation. And secondly because it is not a sufficient explanation of even the every-day things around us if reductionism is false as I believe it is. Explaining why reductionism is false is something I hope to do a lot more work on in future.