For the purposes of this discussion I’m going to ignore that this claim doesn’t follow from the premise. Even if everyone around me don’t have minds, it doesn’t follow that there is no basis for ethics.
We can’t however establish true belief in other minds by evidence.
True belief? No, we can’t establish that. But you can’t do that with a “properly basic belief” either. That’s just another word for an assumption you can’t demonstrate the truth of.
But the entirety of ethics already rests on that, an assumption. We can’t derive an ought from an is, we have to assume it. We don’t know that others have minds, we assume it.
This is another example of those things physicalists are scolded for by people who can’t solve the problem either. You really think people are fooled into thinking that the existence of other minds is a “truth” by having it called a “properly basic belief”?
We have a proper basic belief in other minds, but we can’t establish this belief with evidence.
Hence it isn’t knowledge, nor even is it known to be “true”. It’s an assumption.
Though it might not be. Everyone I know profess to have minds. Now the question is if that is more likely on the hypothesis that they actually do have minds, than if they don’t?