What scientists now agree on
For most of the twentieth century, serious researchers avoided the question of animal minds. Attributing feelings to animals looked like sentimental anthropomorphism, and behaviorism treated even human consciousness as off-limits. That has changed. In 2012 a group of neuroscientists issued the Cambridge Declaration on Consciousness, stating that many non-human animals, including all mammals and birds and creatures such as octopuses, possess the neurological substrates that generate consciousness. Their striking conclusion: the absence of a neocortex, the folded outer brain layer humans rely on, does not prevent an organism from having affective states like pleasure and distress.
The consensus has since widened. In April 2024, dozens of leading scientists and philosophers signed the New York Declaration on Animal Consciousness, organized by Jeff Sebo, Kristin Andrews, and Jonathan Birch. It holds that there is strong scientific support for conscious experience in other mammals and birds, and at least a realistic possibility of it in all vertebrates (reptiles, amphibians, and fishes) and many invertebrates (at minimum cephalopod mollusks, decapod crustaceans, and insects). The signatories add an ethical point: when conscious experience is a realistic possibility, it is irresponsible to ignore that possibility in decisions affecting the animal.
How you study something you cannot feel
Consciousness here usually means sentience: the capacity to have experiences, to feel something rather than nothing. Because no one can climb inside another creature's head, scientists look for converging evidence. They study behavior (does the animal weigh costs against rewards, learn to avoid harm, seek relief?), physiology (are there pain-sensing nerves and natural painkiller systems?), and neuroanatomy (are there brain structures that integrate information the way conscious processing seems to require?). No single test proves an animal is conscious, but when many independent lines of evidence point the same way, confidence grows.
This approach has produced surprises. Bees can learn abstract rules and show behavior that looks like optimism or pessimism. Cleaner wrasse fish pass a version of the mirror mark test once thought to signal self-awareness. Cuttlefish appear to remember specific past events. None of these findings settles the deeper question by itself, but together they have pushed the burden of proof: the interesting debate is no longer whether any animals are conscious, but how far down the tree of life experience reaches, and what it is like at each branch.
What we still cannot know
In 1974 the philosopher Thomas Nagel argued that even if we knew everything about a bat's brain and echolocation, we still could not know what it is like to be a bat, because experience is tied to a particular point of view. That gap has not closed. We can say with growing confidence that many animals feel; we cannot directly share the quality of what they feel. This is why careful scientists talk in terms of evidence and probability rather than certainty.
The practical upshot is a precautionary stance now shaping law and policy: uncertainty about an animal's inner life is not a reason to assume it has none. Recognizing that animals may be conscious does not require deciding they think exactly as we do. It means taking seriously that the world may appear to them as something, from the inside, and letting that possibility count.