Imagine using stem cells to create a fragment of a human brain in a petri dish. It has no eyes to see, no mouth to speak, and no body to move. It’s just a part of a brain, designed to do one small aspect of what a brain does, like respond to stimulation or generate rhythmic electrical activity. It cannot tell you what it is thinking, what it is feeling, or whether it is thinking or feeling at all. Now imagine that scientists discover that this tissue has developed increasingly complex patterns of neural activity… the kinds of activity associated with complex, conscious brains. Would you handle the brain fragment in the dish differently?
Brain organoids are three-dimensional clusters of neural tissue derived from human stem cells. They are not just passive clumps of cells but have been found to exhibit spontaneous electrical activity, coordinated neural oscillations, and even the ability to record the passage of time. Remarkably, some organoids have developed brain waves resembling those seen in preterm human infants. Organoids are valuable scientific tools that could help researchers explore new, energy-efficient forms of computation, study neurological development, model disease, and test potential treatments without relying entirely on animal or human subjects.
However, as these systems become increasingly sophisticated, questions about their existence and ethics arise. One major question preoccupying scientists, philosophers, bioethicists, and the general public is whether an organoid could become conscious.
To clarify, scientists do not currently believe that brain organoids are conscious, nor is there any established evidence that they possess subjective experience. But even that uncertainty presents a bioethical problem. Conscious states are at least part of what allows an entity to experience pain, pleasure, and sensory phenomena. If consciousness matters morally, at least to some degree, then the possibility of consciousness does too. Determining whether something is conscious could help determine whether our actions toward it are morally permissible.
Brain organoids give us a particularly strange version of this problem. They produce no behavior we could read, only electrical activity measured in the lab. Yet the possibility of consciousness, or of elements of conscious activity, looms.
We already know that something can be conscious without acting conscious. Think of dreaming, or of patients who are paralyzed but fully aware. The harder question is what evidence could ever lead us to attribute experience to something that has no way to act at all.
Philosopher David Chalmers popularized a thought experiment called the philosophical zombie, a hypothetical being that is physically identical to a normal human being, down to the last molecule, and so behaves exactly as we do, but that lacks subjective experience. In other words, it lacks what’s called phenomenal consciousness: the subjective, first-person experience of “what it’s like” to perceive, feel, and think. The zombie could use neuronal activity alone to laugh at a joke or wince when it stubs its toe. However, there is no spark of consciousness animating its actions. This is the hard problem of consciousness: even if neuroscience could explain exactly how neurons fire and how information is processed, why would any of it feel like something? Why is there an inner experience accompanying physical activity?
While Chalmers used the thought experiment to challenge the idea that consciousness can be fully explained by physical processes, the philosophical zombie raises an important ethical question as well. Intuitively, we might think that it’s less problematic to harm a zombie than to harm a human being. After all, it lacks phenomenal consciousness, which by many accounts at least in part explains humans’ moral significance. Inner lives imply that there is someone, not just something, who ought not to be harmed.
Brain organoids may present the opposite possibility. Chalmers’ zombie has everything we associate with a conscious mind, the full brain and the full range of behavior, except experience itself. An organoid has only a fragment of a brain and none of the behavior, yet it may contain some of the biological machinery associated with conscious activity.
In other words, brain organoids are potential reverse zombies that could, in theory, have phenomenally conscious states without any outward indication. Most organoids have no means of acting on the outside world. If an organoid were somehow capable of even the faintest spark of a phenomenally conscious state, it would have no way to tell us. This isn’t a purely hypothetical worry. Some patients with severe brain injuries are aware but cannot move or speak, a condition clinicians call covert consciousness.
This is why the reverse zombie is a bioethical problem. Consciousness is fundamentally a first-person phenomenon. We only have direct access to our own inner experience, and can never directly observe the inner life of others. Instead, we look for consciousness from the outside. We observe behavior and make inferences. A person might speak, react emotionally, solve problems, and describe their thoughts. These outward behaviors provide evidence that an internal experience accompanies them.
Brain organoids, on the other hand, give us a potentially silent mind. There may be nothing to observe because there is no body to express it. The philosophical zombie was designed to show that behavior alone cannot explain (or prove the existence of) conscious activity. Brain organoids now suggest that behavior may not even be necessary to raise questions about consciousness in the first place.
It might be tempting to dismiss the reverse zombie as a philosophical thought experiment. After all, there is currently no good evidence that brain organoids are conscious. Why worry about the suffering of something that may have no capacity to suffer? Because as science progresses, we will need to determine whether an organoid can have experiences and identify the evidence that would convince us that it does. We’ve been there before. People who cannot speak or move, from non-speaking people with profound intellectual disabilities to patients diagnosed as vegetative, have often been assumed to have no inner life because they couldn’t show one. Brain imaging has since shown that some patients diagnosed as vegetative could follow instructions. Our tools for detecting minds have a track record of missing them.
The question is also harder than it looks. Phenomenal consciousness isn’t just valenced phenomenally conscious states (the goodness or badness that characterizes some subjective, first-person experiences). It also encompasses the experience itself, of seeing red, tasting something sour, or perceiving electronic stimulation, whether or not that hurts or feels good. It might turn out that the very capacity to have a phenomenally conscious experience is morally significant, or… it might not. And even if an organoid could feel something, it would remain an open question whether that capacity alone makes it the kind of being we owe moral consideration.
This does not imply that we should stop organoid research. The potential computational, scientific, and medical benefits are enormous. Consciousness is only part of the puzzle, and there are active debates about whether and how consciousness confers moral status. Instead, we suggest that two questions deserve far more attention than they get: what should count as evidence of conscious activity in a being that cannot act, and why such evidence would change what we owe it.
Ally Peabody Smith, PhD, is an assistant professor at Lehigh University’s College of Health.
Hannah Gonzales is a student at Lehigh University double-majoring in Cognitive Science and Health, Medicine, and Society.