One of the most famous letters of philosophy was written by Dubliner William Molyneux on July 7th, 1688. It was addressed to the enlightenment thinker John Locke and posed a question that would become known as Molyneux's Problem.

Suppose a man was born blind and had learned how to distinguish a globe from a cube by holding them in his hands. If his sight was suddenly restored could he then distinguish the globe from the cube by looking upon them without touching?

“That was a good question,” says Barry C Smith, who heads up the Centre for the Study of the Senses at University of London. Various answers have been given down the centuries by philosophers and scientists, but there’s still no consensus.

What makes the question so good is that it forces people to rethink their assumptions about perception. Many philosophers have assumed perception is merely sight – a “bizarre” notion, says Smith, as the evidence suggests that touch, sight, smell and the other senses interact in a complex way to produce our lived experiences.

Originally a philosopher of mind and language, Smith became more of an empiricist through what he called “an amateur interest” in wine. “There I was telling people about the taste of wine, and then I thought: How does taste actually work?” He discovered many flavours came not from the tongue but from the nose, and that texture and presentation played key roles.

“It’s something you can demonstrate for yourself,” Smith says. Try eating a jelly bean with your nose pinched and all you’ll get is a shot of sweetness. Let go of your nose and suddenly the fruit flavours appear.

There’s a lesson here for philosophers, says Smith. “Armchair” thinkers who eschew the discoveries of biology and neuroscience are “missing fundamental facts about our experience”, he says.

“If you start from the wrong construction of the phenomena, then you might produce a very clever piece of philosophy, but it will be worthless because it’s not actually getting to grips with how things really are.”

Aristotle said there were five senses – smell, sight, touch, taste, and hearing – but science suggests there are many more than that. How many exactly?

Barry C Smith: “There could be anything between 22 and 33; there is a lot of argument about it. What are they? A sense of balance, for a start, that’s hugely important; if that goes wrong your perceptual world is really in a mess.

“If you close your eyes now you know where all your limbs are without looking at them or touching them; that’s proprioception. Clearly it’s a sense.

“You’ve also got somewhat unusual senses. The sense of effort: if you go to lift a cup of tea but somebody had replaced it with a perfectly good replica in polystyrene you would just throw it over your shoulder. There is a sense of effort needed to actually heft something, to lift it or manipulate it.

“There is also the sense of agency: when you reach for the cup you think ‘I reach for the cup’. People who have neurological damage will sometimes find their arm going out and picking up a cup and they say ‘I didn’t do that’, and of course, they did, their brain is executing that manoeuvre but if they’ve lost their sense of agency... it doesn’t feel like their action.

“Once you start to add all these together you realise there is a symphony of senses, and they are often so well-orchestrated with each other that we don’t notice their operation. We don’t realise they are permanently at work.

“The example I really like of how we get our experience wrong is where you are on an aeroplane on the ground and you get strapped in and you look along the cabin, and see where everything is, when they are giving the safety instruction.

“Look along the cabin again when you are in the climb and it will now look to you as though the front of the cabin is higher than you are. How can it look that way, because you are in exactly the same visual, optical relation to everything in the cabin?

“So it’s not a pure visual experience. It’s being created by the fluid in your ear canals when you tilt backwards, telling you that you are now tilting backwards, and this influences your vision and changes what you see.”

How does all this put philosophy in a bad light?

“Well, I think something that bothered me as someone interested in taste, and therefore smell, is that you look at a lot of philosophy textbooks and the great thinkers of the past, and they talk about a theory of perception, and they say: ‘Take the case where I am looking at a tree, or cup…’, and then they think ‘I’m going to build my theory of perception around that’. What they are talking about is a theory of vision.

“And many of them think once you get the account of vision right then all you have to do is make some modifications and you’ll be able to explain how the other senses work, and the other bits of perception will fall into line. And I think that’s just not right.

“The more I look at taste, touch, smell and the other bodily senses the more I think vision is the odd one out. It’s quite unlike anything else. It presents us with a permanent visual scene. We maintain a scene around us in a way you don’t with smell, and you don’t always have with hearing. You can turn things off, you can fail to attend.

“With touch, think of the clothes just now on your skin; you’re not maintaining a permanent sensory, receptive field of how things feel touching you. You don’t need to.

“But vision keeps you in the present surroundings you are looking at, and that’s just not like any other sense. So I think it would be a mistake to model perception on vision, and philosophers almost don’t notice it.

“Nowadays you see certain questions that seem philosophically hard but when you look at the empirical neuroscience results you realise those questions are not so hard to answer, or maybe they are not the key questions.

“Philosophers like to say: You look at a cup on your desk and you only see it from one point of view - you don’t see the other side; so how do you see that as a three dimensional object? Maybe it’s an illusion, and there’s nothing round the back? And so on.

“But most of the time when we have something of that size we have our hand around it. So we are getting visual and tactile information that the brain is fusing together. When you see an object that you have handled your brain is already partly completing the expectations of touch.

“It’s not hard, if you think of the cup as something you would see and handle and feel, to think how you would perceive the world as having three dimensional objects and not just surfaces that are visually available to you.”

Does this have an impact on scientific observation? Should we doubt the veracity of what scientists report, as all visual evidence is arguably incomplete?

“I think philosophers sometimes have a slightly naive view of scientific theorising. They think you look down the microscope or telescope and you see something and that gives you the result.

“Most modern science is a matter of having many sources of evidence and doing interpretations of those results. The interpretations, statistics and modelling behind it are hugely complicated and contested, and there is something quite active and creative in deciding what the right view is.

“So in a way scientific method, using many different targets and also many different hooks into the same subject matter, and then trying to triangulate them, is what our perceptual system does. It is what the senses do: they take multiple sources of information and then sum up what’s going on out there.”

Have you encountered resistance from fellow philosophers to your line of argument?

"Huge resistance. I find that puzzling. Why? Because the people who say to me 'You are not doing philosophy, you are not being pure, you look more like a sensory scientist', their heroes are often people like Locke, Berkeley, Hume, Descartes and so on. These are people up to their ears in the science of the day; they did science.

“Locke published papers on the trigeminal nerve. He writes treatises on agriculture. Descartes was cutting open animals and wrote about optics, finding out that you had a reverse image on the lens from the retina and so on. So it’s very funny that the purists are often forgetting that the people who seem to do the philosophy they most admire actually talk about the science.

“I think the worry is: am I just going to accept scientific answers to philosophical questions? And the answer is no. I don’t think, for example, neuroscience is going to answer philosophical questions. But I think it is going to help us understand the phenomena, and the kind of evidence we must draw on, when posing those questions or framing them in the right way.”

ASK A SAGE

Question: Should philosophers tweet?

John Locke replies: "To think often and never to retain it so much as one moment is a very useless sort of thinking."