How would we know if an animal is conscious?

Nobody has a single test. Consciousness cannot be seen from outside, so scientists combine several kinds of evidence: what an animal does, what its brain is doing, and, for humans only, what a person says. In humans there are three common methods of studying consciousness: verbal reporting, behavioural demonstrations and neural correlation with conscious activity, though these can only be generalised to non-human animals with varying degrees of difficulty.

Behaviour can be clever. In a study on rhesus monkeys, Ben-Haim and colleagues used an approach that predicted opposite behavioural outcomes for aware and unaware perception, and the monkeys showed the same opposite outcomes as humans. The brain can be examined too. The neural correlates of consciousness are the minimal set of neuronal events and mechanisms sufficient for a specific conscious percept. Researchers once argued that consciousness arises in the neocortex and so cannot arise in animals without one. That view has been challenged, and many researchers now believe animal consciousness can arise from subcortical networks.

The oldest answer was no test at all. Descartes believed that humans alone possess a non-physical mind and interpreted animal behaviour through a mechanistic model that denied animal consciousness. The argument has not gone away. The writer Eugene Linden points to animal behaviour and intelligence that surpass what people would suppose to be the boundary of animal consciousness, while the philosopher Daniel Dennett counters that consciousness is not all-or-nothing and that the differences between humans and other species make speculation about animal consciousness seem ungrounded.

A newer idea is to stop asking yes or no. A 2020 paper proposed that consciousness may not be meaningfully represented along a single line, and suggested five dimensions for building a profile of each species instead.