The argument is trying to say that the distinction from "actual reality" (whatever that means) and perception of reality doesn't make much sense. One cannot break free of their perceived reality in order to see/compare/reason about "actual reality".
> And if the counter-argument is that the perception of the outsiders is what causes those objects to exist ...
As I understand it, this is Berkeley's position.
> ... if we then killed all of those outside observers (say, just a few researchers who are aware of what is in the room surrounding the child's room), would the objects in that room then cease to exist (and if so, by what mechanism that we reasonably know exists)?
Yeah, basically -- assuming you, the experimenter/person who is observing this thought experiment, is also killed.
I think the issue with some of these thought experiments is that they assume there exists some omniscient perspective who can see every part of the experiment (i.e. the person running the experiment).
> To me, the obvious flaw here is that there seems to be an implicit "only" perceived within the conclusion: "Ordinary objects are [only] ideas [and nothing else]." This is an extremely common error that the human mind makes, but you'd think that a philosopher would catch it in review of an idea (or the reviewers, who say: "The argument is valid, and premise (1) looks hard to deny."), so I feel like I must be missing something in the argument.
Yeah, I think the link I sent doesn't sum up Berkeley's point well. Perhaps it'd be better to take a look at the original argument. [1]
> > Does it really make sense when materialists argue that unobservable, undetectable, totally unperceivable and uninferrable things in this world actually exist?
> It makes complete sense to me (and I typically disagree with materialists)!
Interesting! How do you reconcile this with the belief that "reality is contingent on our minds"?
[1]: This was the best-formatted online link I could find: https://www.marxists.org/reference/subject/philosophy/works/... (part 1, sections 3-5 are most relevant)