The proxies must resemble actual brains to yield insights that could improve human lives; but the better the resemblance — that is, the closer they come to consciousness — the harder it is to justify using them for our selfish purposes.

“If it looks like a human brain and acts like a human brain,” writes Stanford law professor Henry Greely in an article published in The American Journal of Bioethics in 2020, “at what point do we have to treat it like a human brain — or a human being?”

A big part of the problem is that we can’t answer that last question — there isn’t a clear way to tell if an organoid is suffering. Humans and animals use their bodies to communicate distress, but a blob of neurons has no means of connecting with the outside world.

Greely, who specializes in biomedical ethics, puts it grimly in his 2020 article: “In a vat, no one can hear you scream.”