Are animals conscious? Notoriously, the famous 17th century philosopher René Descartes thought they were not. He believed that possession of a soul was necessary for rational thought and for consciousness, including the capacity to feel pain and pleasure. He therefore believed that there was nothing inherently wrong with kicking or cutting animals. Even though they would engage in pain behaviour (crying, shrinking back, attempting to escape) they would not actually experience pain.
In the 1970s and earlier, it was common to hear scientists claiming that it was mere sentimentalism to attribute the capacity for suffering to animals. However, even then it was clear that there was little reason to agree with them. So the recent Cambridge Declaration, signed by leading neuroscientists, that “the weight of evidence indicates that humans are not unique in possessing the neurological substrates that generate consciousness” really should come as no surprise.
There are three possible reasons why a scientist might once have thought that animals can’t be conscious. First, they might agree with Descartes and with many other thinkers steeped in a Christian worldview that animals lack a soul. However, the postulation of a soul is no part of science - there is no scientific reason to think that we have any kind of non-physical properties that animals lack.
Second, they might subscribe to behaviourism, according to which there is no need to postulate internal mental states to explain behaviour. But behaviourism, once influential, has now almost entirely vanished because rival perspectives that explain behaviour by reference to internal mental states have been so much more successful.
The third reason why a scientist might deny that animals are conscious is that unlike adult human beings, they can’t tell us (in language) that they’re conscious. That, I suspect, was the most popular reason why some scientists were holdouts for so long. But it really isn’t a very good reason. For one thing, lots of human beings can’t tell us that they’re conscious – infants, and people who are paralysed, for instance. There is little temptation to think that the ability to feel pain is lost along with the ability to tell other people about it. For another thing, pain behaviour seems at least as good an expression of pain as does language (and harder to fake).
The problem of course is that the word “consciousness” is multiply ambiguous, and different kinds of consciousness have different ethical implications.
“Consciousness” is probably most frequently used today to refer to what philosophers often call phenomenal consciousness. This is the sense of consciousness in which pain is conscious: we are phenomenally conscious of the taste of wine, the warmth of the sun on our face, of the blueness of the sea, and so on. The badness of pain consists largely (though perhaps not entirely: after taking certain drugs people report that pain still feels the same way but they don’t mind it anymore) in the way it feels. So if animals are phenomenally conscious, then it matters how we treat them and whether we inflict pain and suffering on them.
When Freud talked about the unconscious, however, the conscious/distinction he had in mind was a different one. In that sense of “consciousness”, a state is conscious when we access it; typically, if we are conscious of something, in that sense, we can report it.
Consciousness scientists disagree with one another whether the two kinds of consciousness always go together, or whether they can sometimes come apart. This matters, because this kind of consciousness is probably required for the kind of psychological complexity that many philosophers think is needed to have a life worth living - to think of oneself as an being persisting in time, with plans and projects of one’s own.
It follows that if animals are phenomenally conscious but lack this kind of consciousness, it would be wrong to cause them pain (that is, we could only cause them pain if we had a good justification), but not wrong to kill them painlessly.
A third sense of consciousness is also relevant here, and that is of the self. Scientists have a variety of tests for self-consciousness. The most famous is the mirror test - a dot is painted on the forehead of an anaesthetised animal and its reactions when it sees itself in a mirror are studied. Does it rub the spot or contort its body to get a better view, indicating that it recognises that the reflection is itself? Only a few animals pass this test: great apes and dolphins, almost certainly, some other primates more controversially. Again, this seems good evidence that only some animals have the kind of consciousness that makes painless killing of them wrong.
The Cambridge declaration represents a public acknowledgement of what most scientists have known for decades now: non-human animals are more like us than we typically suppose. They are alike enough that it is clear that inflicting pain and suffering on them requires justification. The evidence is growing that many of them are alike enough to have lives worth living, so we require good justifications for killing them.
Exactly which species have lives worth living, and exactly how good a justification we require for killing them, requires further scientific and philosophical research. But we already have good grounds for thinking that we tend to be cavalier in our treatment of animals. Public attitudes lag well behind the scientific consensus.
Ironically, research on animals raises more public concern than their casual slaughter for food (google UK animal activists for some disturbing evidence), but that very science is showing us that concern should be broader than it is.