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.