THERE is a shelf in my brain – and in my study too – where I store theories of consciousness, and this is where psychologist Nicholas Humphrey’s new book, Sentience, will reside. Each of these theories and hypotheses can seem compelling in the moment, especially given the confidence with which they tend to be presented. They are usually unlike each other; often, they are diametrically opposed.
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For example, in Sentience, Humphrey invokes children born with hydranencephaly, a condition where the brain’s cerebral hemispheres are absent, replaced by sacs filled with cerebrospinal fluid. These hemispheres control key areas like muscle function, speech, thought, emotion and learning. Neuroscientist Mark Solms also invokes such children in his book The Hidden Spring. For him, the children’s responses to their environment imply that consciousness is lodged in the brain stem. For Humphrey, it shows the opposite. And so on.
I don’t mean to be dismissive. Sentience is full of provocative ideas, as well as lively anecdotes from decades of pondering these issues. Humphrey’s thesis offers a great deal to think about. But it doesn’t persuade any more than the others.
Even the terminology is contentious. Humphrey considers sentience to be a “higher” form of consciousness than what he calls cognitive consciousness, in which the mind is merely aware of stimuli that influence behaviour; in sentient “phenomenal consciousness”, these perceptions are accompanied by subjective feelings philosophers call qualia, perhaps the biggest battleground of the philosophy of mind. But some others working on these problems consider sentience as a primitive form of consciousness, a mere buzz of awareness.
Humphrey’s distinction is illustrated by the phenomenon of blindsight, in which people with a damaged visual cortex who don’t report “seeing” objects still respond to them because the retina has registered them. In fact, we respond to all manner of bodily sensations without representing them phenomenally: we are cognitively, but not phenomenally, conscious of them.
Once, argues Humphrey, all consciousness might have been only cognitive: senses informed actions, but their signals weren’t presented to the self along with corresponding qualia. He implies that most non-human animals, at least up to the complexity of frogs, might have minds like this. In truth, we have little idea if that is the case.
Humphrey denies sentience even to octopuses, writing that the likelihood of their being sentient and having a phenomenal self is “negligible”. He adds later: “If you were in the place of a sentient dog, you would mind about being treated well by humans; if you were in the place of an insentient octopus, you wouldn’t.” On what basis does he argue this? In part, he thinks only sentient animals “play” – but it isn’t clear what counts as play, and it has been posited that octopuses and even insects engage in it. Humphrey’s arguments are less than compelling, so the assumption is morally contentious.
Wherever it begins, phenomenal consciousness changes everything. “It’s an invention so sublime,” Humphrey writes, “that, if it were to cease to exist, it would indeed diminish the whole of creation.” Having it, we have a duty to preserve it.
If this is so, why did evolution invest in giving us so much feeling about our world? Humphrey insists it must be adaptive, and may be advantageous to creatures who engage in complex sociality.
I am not so sure. What’s more, there is no consensus on whether our consciousness is adaptive, and, however important it seems to us, no strong argument why it could not be primarily a neutral adaptation. Humphrey doesn’t consider this, though it has been proposed that much of the complexity in higher animals arose this way.
So I fear Humphrey provides no more compelling an explanation of sentience or consciousness, or how they arose, than those other volumes on my shelf.
But his book earns its place among them, and is a valiant reminder of how much there still is to understand.
Philip Ball is a writer based in London. His latest book is The Book of Minds
