Terry Pratchett - The Globe

This story shows how easily we can get sensations wrong. In this case, you know what it vomeronasally smells like to be a human: you don't feel anything at all, not consciously. But you certainly respond! So your reactions, and what they 'feel like', are very different. The sounds we hear, the sensations of heat and cold on our skin, the smells that assail our nostrils, the unmistakable taste of salt ... all these are qualia, vivid 'feelings' stuck on to our perceptions by our minds to help us recognise them more readily. They have a basis in reality, yes, but they are not real features of the outside world. They must be real features of brain architecture and function, real things happening in real nerve cells, but that level of reality is very different from the level that we perceive.

So we should be suspicious of the belief that we can know what it feels like to be a dog. In 1974 the philosopher Thomas Nagel published a famous essay 'What is it like to be a bat?' in the Philosophical Review, in which he made the same point. We can imagine what it is like to be a human who is behaving -superficially at least -like a bat, but we have no idea what it feels like to the bat, and it is questionable whether human knowledge can ever extend in such a direction.

We probably get bats wrong anyway. We know that bats use echo-location to sense their surroundings, much as a submarine uses sonar. The bat or submarine emits sharp pulses of sound, and hears the returning echoes. From those, it can 'compute' what the sound must be bouncing off. We naturally assume that the bat responds to echoes in the same kind of way that we would: it hears them. We naturally expect the qualia of bat echo-location to be similar to the human qualia evoked by sound-patterns, of which the richest example is music. So we imagine the bat flying along to the accompaniment of incredibly rapid rhythms played on bongo drums.

However, this could be a false analogy. Echo-location is the main sense of a bat, so the 'correct'

corresponding sense of a human is its main sense, which is sight, not hearing. The August 1993 edition of Nature has a picture of a bat on the cover, with the words 'How bats' ears see'. This refers to a technical article, by Steven Dear, James Simmons and Jonathan Fritz, who discovered that the neurons in the part of the bat's brain that processes returning echoes are connected together in a very similar way to those in the human visual cortex. In terms of neural architecture, it looks very much as if the bat's brain uses the echoes to build up an image of its surroundings. Analogously, today's submarines use computers to turn a series of echoes into a three-dimensional map of the surrounding water. Figments of Reality developed this point to give a partial answer to Nagel's question:

[In effect] bats see with their ears, and their sonar qualia might well be like our visual ones.

Intensity of sound might come over to the bat as a kind of 'brightness', and so on. Possibly the bat's sonar qualia 'see' the world in black and white and shades of grey, but they could also pick up and render vivid various more subtle features of sound reflections. The closest analogy in humans is texture, which we sense by touch, but the bat could sense by sound. Soft objects reflect sound less well than hard ones, for instance. So bats may well 'see' textured sound. If so and here our analogy is intended only as a very rough way to convey the idea -the sonar quale for a soft surface might 'look' green to the bat's mind, that for hard ones might look red, that for liquid ones like a colour only bees can see, and so on ...

On Roundworld, such statements are no more than guesswork, supported by analogies of neural architecture. On Discworld, witches know what it feels like to be a bat, or a dog, or a beetle. And Angua the werewolf smells in colours, which is very close to our suggestion that bats hear in images and 'see' textures. But even on Discworld, the witches do not actually feel what it is like to be a bat. They feel what it is like to be a human who has 'borrowed' the sensory organs and neural processing equipment of a bat. It may feel quite different to be a bat when a witch is not hitching a free ride on its mind.

Even though we can't be certain what it feels like to be an animal, or another person, the attempt has several uses. As we said, the ability involved here is empathy: being able to understand what another person feels like. We've already seen that this is an important social skill, and that the same ability, deployed in a different way for a different purpose, gives us a chance to detect that someone else is lying to us. If we put ourselves inside their heads and realise that what they are saying is different from what we believe they are thinking, then we suspect them of lying.

The word 'lie' has negative overtones, deservedly so, but what we're talking about here can be constructive as well as destructive, and often is. For the purposes of the present discussion, a lie is anything contrary to the truth, but it's not at all clear what 'the truth' is, or even whether there is only one of it, as the word 'the' would seem to indicate. When two people have a row, it is generally impossible for either of them, or anyone else, to figure out exactly what really happened. Our thoughts are tainted by perceptions. This is unavoidable, because what we think of as being 'real' is what our minds make of what comes from the sense organs: fudged, tuned, and mangled by a succession of interpretations by different bits of brain, plus some wallpaper additions. We never know what is really out there around us. All we know is what our minds construct from what our eyes, ears and fingers report.

Not to put too fine a point on it, those perceptions are lies. The vivid universe of colour that our brain derives from the light that falls on our retinas does not really exist. The redness of a rose is derived from its physical features, but 'being red' is not a physical feature as such. 'Emitting light of a certain wavelength' gets closer to being a physical feature. However, the vivid redness that we 'see' does not correspond to a specific wavelength. Our brains correct the colours of visual images for shadows, light reflected on to parts of the image by other parts of a different colour, and so on. Our sensation of redness is a decoration added to the perception by our brain: a quale.

So what we 'see' is not an accurate perception of what is there, but a mental transform of a sensory perception of what is there.

To a bee, that same uniformly red rose may look very different, with obvious markings. The bee

'sees' in ultraviolet, a wavelength outside our range of perceptions. The rose emits a whole distribution of wavelengths of light; we see a small part of that, and call it reality. The bee sees a different part and responds to it in its own beelike way, using the markings to land on the flower and collect nectar, or to dismiss it from consideration and fly on to the next possibility. Neither the bee's perception, nor ours, is the reality.

In Chapter 24 we explained that our minds select what they perceive in more ways than just passively ignoring signals that our senses can't pick up. We fine-tune our senses to see what we want them to see, hear what we want them to hear. There are more nerve connections going from the brain to the ear than there are from the ear to the brain. Those connections adapt the ear's ability to perceive certain sounds, maybe by making it more sensitive to sounds that could represent danger and less sensitive to sounds that don't really matter much. People who are not exposed to certain sounds as children, when their ears and brains are being tuned to pick up language, cannot distinguish them as adults. To the Japanese, the two phonemes 'l' and 'r' sound identical.

The lies that our senses tell us are not malicious. They are partial truths rather than untruths, and the universe is so complicated, and our minds are so simple in comparison, that the best we can ever hope for is half-truths. Even the most esoteric 'fundamental' physics is at best a half-truth.