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Imagine a woman who has somehow been brought up from birth inside a black, white and grey room, so everything she sees is in monochrome. Yet imagine also that she has spent her life studying the science of colour. She learns how different wavelengths of light are perceived by the eye, how a prism separates white light into a spectrum and so on – but has never personally seen anything other than shades of black or white. Now, imagine that she leaves the room for the first time and sees the vibrant palette of the real world. Most of us would agree that, at that moment, the woman learns something new about colour.
, proposed by philosopher in 1982, was intended to argue against physicalism, the belief that there is nothing over and above the physical universe. But it also suggests that there are types of knowledge that can’t be gained by reading, measuring or deducing. They have to be learned through direct experience.
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This article is part of a special series on the limits of knowledge, in which we explore:
The impossibility of sharing someone else’s subjective experiences has consequences for the world of medicine. It makes it harder to know what is going on when someone has hallucinations, for instance, or to know how much pain someone is in. We rely on their descriptions, with no way to know if one person’s “ache” is another’s “agony”. “It is impossible for me to feel your pain,” says , a philosopher at the University of Oxford. “The idea is that the mind is a private world, hidden behind a kind of super barrier. It isn’t a physical barrier like your skull, because even if we could physically get inside your head, it is impossible for us to breach it.”
Subjective experience
Another consequence of this limit on knowledge is that we can never know if our perceptions of the world are the same as those of someone else. There is much experimental evidence showing that people can have different experiences of particular colours, sounds, smells and so on. This isn’t just because their sense organs may have slight physical differences, but also because their brain cells may process those inputs differently.
This became more widely appreciated in 2015 when social media exploded with arguments about “the dress“: an image of a two-tone garment that some saw as white and gold and others said was blue and black. “It’s hard to understand that other people might see the world differently, [until] you get weird things like the dress,” says , a neuroscientist at the University of Sussex, UK.
Seth and his colleagues are investigating the diversity of human sensory experiences through the , an online survey of how people experience games, illusions and other visual and auditory stimuli. “There are differences that we know about and are nameable – people that are colour blind, for example,” says Seth. “But we want to understand differences that are in the middle of the distribution rather than the tails.”
If it is hard to really know the minds of other people, then it is arguably even harder to get a window into those of other species with completely different sensory inputs (see “What is it like to be a bat?”). Or how about the experiences of an intelligent machine, with a “mind” that isn’t even made of the same basic materials as us – surely that would be harder still to comprehend?
This leads to a serious problem: how will we know if we ever create an AI that is sentient? It may be that we will have to judge a computer’s consciousness not by counting how many processors it has, but simply by whether it gives the appearance of sentience. This may seem rather unscientific, but it is the same method used for judging the sentience of our fellow humans. After all, we cannot know for sure if other people have consciousness like our own, we merely assume it, based on their behaviour.
No matter how well we may be able to understand reality through science, using equations, theories and experimental measurement, there is always going to be a crucial aspect that remains at least partly private and unknowable. “There’s this direct experiential knowledge, which only the organism with a particular brain can have,” says Seth.
WHAT IS IT LIKE TO BE A BAT?
Philosopher considered this question in the 1970s in order to argue that it based on our physical understanding of the world alone. Whatever your view on that issue, it hasn’t stopped people trying to understand things from animals’ point of view. One far out example is the designer Thomas Thwaites, who, in 2016, tried to live as a goat on a hillside. He built a goat exoskeleton and external stomach to help him digest grass – no kidding.
Ultimately, Thwaites and others have found there is a wide gulf between our experience of the world and that of some animals. How are we ever going to grasp what it is like to “see” the world through the lens of echolocation? “You can chop up a bat, you can find out everything about its physical construction down to the last atom, but its private, phenomenal feel is going to elude you,” says Stephen Law, a philosopher at the University of Oxford.
This makes it hard to form judgements about the ethical treatment of animals. And the more different the species is from us, the harder this becomes, says Anil Seth at the University of Sussex, UK. “We might be able to imagine what it’s like to be a monkey more than what it is like to be an octopus. But we probably still get it wrong for a monkey.”
