[yldedly]

My point is that QM and GR make very different claims about what exists. Perhaps it's possible to unify the descriptions. But more likely there will be a new theory with a completely different description of reality.

On small scales, GR and Newtonian mechanics make almost the same predictions, but make completely different claims about what exists in reality. In my view, if the theories made equally good predictions, but still differed so fundamentally about what exists, then that matters, and implies that at least one of the theories is wrong. This is more a realist, than an instrumentalist position, which perhaps is what you subscribe to, but tbh instrumentalism always seemed indefensible to me.

[Maxatar]

If you are aware that "Maxatar's conjecture is that 1 + 1 = 5", then it's correct to say that you have knowledge about "Maxatar's conjecture", regardless of whether the conjecture is actually true or false. Your knowledge is that there is some conjecture that 1 + 1 = 5, not that it's actually true.

In that sense, it's also correct to say that physicists have knowledge of relativity and quantum mechanics. I don't think any physicist including Einstein himself thinks that either theory is actually true, but they do have knowledge of both theories in much the same way that one has knowledge of "Maxatar's conjecture" and in much the way that you have knowledge of what the flat Earth proposition is, despite them being false.'

It seems fairly radical to believe that instrumentalism is indefensible, or at least it's not clear what's indefensible about it. Were NASA physicists indefensible to use Newtonian mechanics to send a person to the moon because Newtonian mechanics are "wrong"?

What exactly is indefensible? The observation that working physicists don't really care about whether a physical theory is "real" versus trying to come up with formal descriptions of observed phenomenon to make future predictions, regardless of whether those formal descriptions are "real"?

If someone choses to engage in science by coming up with descriptions and models that are effective at communicating with other people observations, experimental results and whose results go on to allow for engineering advances in technology, are they doing something indefensible?

[yldedly]

Yes, it's correct to say that I have knowledge of your conjecture, and in the same way that physicists have knowledge of QM and GR regardless of their truth status, but beyond just having knowledge of the theory, they also have knowledge of the reality that the theory describes.

>Were NASA physicists indefensible to use Newtonian mechanics to send a person to the moon because Newtonian mechanics are "wrong"?

No, it was defensible, and that's exactly my point. Even though they didn't believe in the content of the theory (and ignoring the fact that they know a better theory), they do have knowledge of reality through it.

I don't think instrumentalism makes sense for reasons unrelated to this discussion. A scientist can hold instrumentalist views without being a worse scientist for it, it's a philosophical position. Basically, I think it's bad metaphysics. If you refuse to believe that the objects described by a well-established theory really exist, but you don't have any concrete experiment that falsifies it or a better theory, then to me it seems like sheer refusal to accept reality. I think people find instrumentalism appealing because they expect that any theory could be replaced by a new one that could turn out very different, and then they see it as foolish to have believed the old one, so they straight up refuse to believe or care what any theory says about reality. But you always believe something, whether you are aware of it or not, and the question is whether your beliefs are supported by evidence and logic.