“There are many issues why psychiatric disorders have been hard to crack, but one is that we don’t have access to the functioning human brain,” says Sergiu Pasca, a neuroscientist at Stanford University. Imaging technologies are still relatively crude. Animal brains are smaller than ours, and grow differently. Organoids, though also limited in their own ways, “allow us to capture aspects of human brain development that were previously inaccessible,” says Pasca. They might even allow scientists to replace brain tissue that’s been lost through injury or disease.

But Gage’s experiment, which was first announced at a conference last November, shows that the ethical discussion around organoids has yet to catch up with the fast-moving technology for creating them. They are getting bigger and more complex. They’re being transplanted into animals. “It does raise visceral reactions and for this research to progress, the public must be comfortable with it and understand it,” says Nita Farahany, a bioethicist at Duke University. “At some point, these models could become so good that they could approximate the whole brain. The closer they get to the human brain, the sharper the ethical issues.”

At what point would an organoid be worthy of moral status? Of respect? “At what point is it reasonable to at least discuss the question of sentience? Or conscious experience? Pain? Pleasure?” asks Christof Koch from the Allen Institute, “We’re not there, but we have to start thinking about these possibilities.” Farahany, Koch, and 15 other leading ethicists and neuroscientists have now outlined some of those issues in a new paper, based on a workshop they held last May. (The debate was vigorous: “Every one of the authors wordsmithed every line of this paper,” Farahany says.)

Over the last decade, scientists have managed to coax stem cells into many different kinds of tissues and organs. And along the way, they started realizing that these cells have an incredible capacity for self-organization. You don’t need to carefully orchestrate every step of their growth; you just need to give them oxygen, nutrients, the right molecular triggers, and some kind of scaffold so they make the right shapes. And voila—organoids. There are heart organoids, liver organoids, gut and kidney and pancreatic organoids—none of which pose many ethical conundrums.

That changed when, in 2012, Jürgen Knoblich from the Austrian Academy of Science fashioned the first brain organoids. “The brain is what people most closely identify with self and personhood, with the capacity for consciousness and agency,” says Farahany. So the act of duplicating the brain, even in part and even imperfectly, cannot help but create an instinctual unease.

Brains don’t work in isolation, though. They exist within bodies. They come wired up to eyes, ears, and other sensory organs, which provide inputs that are vital for wiring the brain correctly. “An animal deprived of visual inputs never learns to see,” says Koch. “And organoids don’t have any inputs. They won’t see or hear or smell. They can’t recall anything because there’s nothing to recall. They can’t think in any way, shape, or form.”