Sentience without borders: ethics beyond the brain

John Goldie is a retired GP and medical educator

The concept of sentience is central to ethical debates, particularly around the issue of rights and moral status. Understanding which beings are sentient, helps determine who deserves our moral concern, shaping laws, policies, and everyday choices.

Beyond brains: Sentience in simple life forms

Sentience is often confused with consciousness, but the two are distinct. While consciousness includes higher-order functions like reasoning and self-awareness, sentience refers specifically to the ability to have subjective experiences, such as pain or pleasure. Recent research suggests that even simple organisms—microbes, single-celled life forms, plants, and fungi—may possess rudimentary mechanisms for sensing and responding to their environments.^1,2 Though these reactions may not mirror human experience, they challenge the assumption that subjective awareness is exclusive to complex brains.

This idea aligns with evolutionary biology, which shows that complex organisms emerged from simpler ancestors. If basic forms of experience are embedded in life’s earliest stages, then sentience may be more widespread than previously thought.

Artificial minds and alien life: The new frontier

Emerging technologies like artificial intelligence (AI), bioengineering, and synthetic biology are creating novel entities with anatomies and control systems unlike anything found in nature.³ These developments, coupled with the possibility of extraterrestrial life, are prompting a re-evaluation of which beings merit moral consideration.
Traditional views that prioritize human-like traits are being challenged. Sentience may manifest in unfamiliar ways, and our ethical frameworks must adapt to recognize and respect these differences.

Measuring the unmeasurable

One of the greatest challenges in studying sentience is its invisibility. Internal experiences are inherently private, making them difficult to measure directly. In humans, scientists rely on verbal reports, behavioural cues, and neuroimaging data. For animals, researchers infer mental states from observable behaviours.
Efforts to extend this approach to invertebrates have led to frameworks like Crump et al.’s criteria for sentience, which influenced the UK’s Animal Welfare (Sentience) Act of 2022.⁴ These criteria include nociception, sensory integration, memory-based protection, and preference for pain relief (see Fig 1). However, critics argue that relying solely on human-like behaviours risks overlooking other valid expressions of sentience.⁵

Fig 1: Crump et al.’s Criteria for sentience in invertebrates

1. Nociception – Detecting harmful stimuli.
2. Sensory Integration – Organizing and interpreting sensory input.
3. Integrated Nociception – Using memory and learning to avoid danger.
4. Analgesia – Modulating behaviour in response to pain relief
5. Motivational Trade-offs – Weighing competing needs.
6. Flexible Self-Protection – Adapting to protect itself.
7. Associative Learning – Connecting distinct events or stimuli.
8. Analgesia Preference – Seeking pain relief.

Philosophical challenges

The scientific ambiguity surrounding sentience is mirrored in philosophy, where longstanding debates challenge the boundaries between mind and matter. Dual-aspect monism, for instance, proposes that mental and physical phenomena are two expressions of a single underlying reality—suggesting that consciousness may not be confined to biological brains. Panpsychism goes further, positing that consciousness is a fundamental feature of all matter, from electrons to ecosystems.

While these theories remain speculative, they offer provocative alternatives to the dominant view that sentience arises only from complex neural architecture. Importantly, they invite us to consider whether artificial systems—despite lacking organic substrates—might still participate in forms of experience. As we confront increasingly sophisticated AI and synthetic organisms, these philosophical frameworks may help us rethink what kinds of entities are capable of feeling, perceiving, or suffering—and what ethical obligations that entails.

Sentience in machines: The Turing trap

When evaluating artificial systems, we often default to human-like behaviours—speech, writing, emotional mimicry—as indicators of sentience. The Turing Test, which measures a system’s ability to produce human like responses, formalizes this bias: if a machine can convincingly imitate human interaction, we tend to treat it as having a mind. But this approach has limitations. Sentience may not resemble human experience at all and focusing only on familiar traits risks excluding valid forms of awareness.

To ensure fairness, we must assess behaviour consistently across all systems, regardless of their origin or makeup. This shift helps us avoid anthropocentric bias and recognize sentience in its many possible forms.

Ethical implications: Who deserves moral concern?

As our understanding of sentience evolves, so too must our ethical boundaries. If new forms of life or intelligence can sense, respond, or even experience their environments in ways we do not yet comprehend, our current frameworks for moral status may be too narrow to accommodate them.

Granting rights or protections to animals, artificial beings, or unfamiliar forms of life could help prevent unnecessary harm. But doing so takes courage, humility, and a willingness to question our deepest assumptions. Philosopher Peter Singer argues that moral concern should not depend on species, but on the ability to suffer.⁶ Yet suffering might appear in ways we do not yet understand. Martha Nussbaum’s capabilities approach reminds us that flourishing is not one-size-fits-all—it depends on the kind of being and its context.⁷ To navigate this uncertain terrain, we need epistemic humility: the awareness that sentience might emerge in unfamiliar forms, and that the real moral danger lies in failing to imagine what it is like to be ‘other.’

In a world where minds may emerge from silicon as well as cells, the question is no longer just who deserves moral concern—but how we will recognise and respond to sentience wherever it arises.

References

1. Rouleau N, Levin M. The multiple realizability of sentience in living systems and beyond. eNeuro. 2023;10(11).
2. Calvo P, Sahi VP, Trewavas A. Plants are intelligent. Here’s how. Ann Bot. 2020; 125:11–28.
3. Levin M. Generalizing frameworks for sentience beyond natural species. Animal Sentience. 2022;429.
4. Crump A, Browning H, Schnell A, Burn C, Birch J. Sentience in decapod crustaceans: A general framework and review of the evidence. Anim Sentience. 2022;32(1).
5. Butlin P. Sentience criteria to persuade the reasonable sceptic. Anim Sentience. 2022;437.
6. Singer P. Animal Liberation Now. New York: HarperCollins; 2023.
7. Nussbaum MC. Women and human development: The capabilities approach. Cambridge: Cambridge University Press; 2000.

Featured photo by Steve Johnson on Unsplash.