Esse est percipi – to be is to be perceived. That is Berkeley’s great insight, that the world as we know it exists only for us and beings similarly equipped. It is an observation widely misunderstood because the truth of it is difficult to express, hence the famous exchange between my countryman Boswell (whom I could never take to) and Dr Johnson (whom I much admire):

After we came out of the church, we stood talking for some time together of Bishop Berkeley’s ingenious sophistry to prove the nonexistence of matter, and that every thing in the universe is merely ideal. I observed, that though we are satisfied his doctrine is not true, it is impossible to refute it. I never shall forget the alacrity with which Johnson answered, striking his foot with mighty force against a large stone, till he rebounded from it — “I refute it thus.”

This is typically tendentious from Boswell, who has plainly made up his mind – ‘ingenious sophistry’ ‘merely ideal’ and ‘we are satisfied [by what means, one may ask?] his doctrine is not true’ and of course Johnson’s response proves only that he had no more idea than Boswell did what Berkeley was talking about.

The good Doctor’s error is excusable, however, since the problem lies with language and what happens to it when we start to use it for philosophy: words are twisted out of their usual meaning, pushed to their limit till they crack. This must happen, since philosophy is about clarifying our ideas, which in turn involves trying to clarify the language in which we express them – when I was young, I used to think that language could be ‘improved’ and ‘purified’ by philosophy, so making it a fitter vehicle for thought, but now I see that the very murkiness of language, its openness to a range of meanings, its vagueness and ambiguity, are strengths rather than weaknesses.

(Wittgenstein was aware of how odd philosophical discussion could seem to the outsider:

I am sitting with a philosopher in the garden; he says again and again “I know that’s a tree”, pointing to a tree that is near us. Someone else arrives and hears this, and I tell him: “This fellow isn’t insane. We are only doing philosophy.” (On Certainty, 467))

Johnson’s error is repeated by those who think that what Berkeley is saying is that when you leave a room, all the furniture ceases to exist. This seems absurd and impossible to them because they continue to imagine the room, but see it as suddenly empty when the person leaves – something you might illustrate amusingly in an animated film, maybe. ‘But how can the furniture disappear from the room?’ they demand. But the furniture doesn’t disappear from the room: the room is as much dependent on our perception as what is in it. In one sense, Berkeley is saying something we all believe but find unremarkable: you can only see a thing when you’re looking at it.

When you put that in the passive – ‘a thing can only be seen when there is an observer’ you edge towards the real truth of Berkeley’s insight, which is that there is an element in all our observations which we take for granted, namely the observer. You cannot think yourself out of the picture: if you assert, ‘but I know perfectly well what my room looks like when I’m not there – I can picture it now’ then all you are saying is ‘I can imagine what it looks like when I am there.’

A modern-day Johnson might rig a camera with an automatic switch and triumphantly flourish the resultant picture of an empty room, complete with furnishings, exactly as we would expect, saying ‘I refute it thus!’ But to get at what Berkeley meant, you must try to imagine the room (or anything at all) as it appears from no particular point of view. It is impossible, of course, though you might have fun trying to simulate it with a battery of cameras in the same way that a single camera simulates our being in the room when we aren’t. The resultant picture would be a curious one: you would have a simultaneous view of the room and its contents from all four, or rather all six, sides. Perhaps the nearest thing to it might be a cubist painting, such as Georges Braque’s Violin and Candlestick:

Another way of coming at this is to imagine some improvement in the eye, so that our visual spectrum is extended – things that presently seem to us empty space would perhaps be intriguingly shaded and coloured, as described by Old Mathers in Flann O’Brien’s The Third Policeman:

‘There are four winds and eight sub-winds, each with its own colour. The wind from the east is a deep purple, from the south a fine shining silver. the north wind is a hard black and the west is amber. People in the old days had the power of perceiving these colours and could spend a day sitting quietly on a hillside watching the beauty of the winds, their fall and rise and changing hues, the magic of neighbouring winds when they are inter-weaved like ribbons at a wedding. It was a better occupation than gazing at newspapers. The sub-winds had colours of indescribable delicacy, a reddish-yellow halfway between silver and purple, a greyish-green that was equally related to black and brown. What could be more exquisite than a countryside swept lightly by cool rain reddened by the south-west breeze?’

The thing that Berkeley wants us to grasp is that if you think yourself out of the picture, it ceases to be a picture: it is no longer seen from any particular viewpoint, there is no cunning apparatus, be it eye or camera, to detect the visible spectrum of light and give objects in the room shape and colour; if the bonsai tree on the desk keels over, there is no ear to convert the compressions and rarefactions of the air that result into sound; without a nose, the pungent aroma of yesterday’s kipper is no more than particles in the atmosphere.

Picture this: the last man in the world and his dog are gazing out through a triple-glazed window at some apocalyptic scene – meteorites raining down on the earth, perhaps. The last man keels over and dies; since his dog sees in black and white (though he inhabits a rich world of scent and sound of which his late master was almost wholly ignorant) has all colour now gone out of the world?

Put this way, it sounds like something amazing (again, we could illustrate it in animation – as the man is dying, the colours slowly fade to sepia) but that is because of what we have chosen to focus on: say instead that when the last man dies, the world as we have known it dies with him – that brings home to us how much more there is to our world than just the single element of colour: there is memory, emotion, the record of all that small section of sensory data which we are equipped to process, together with the feelings it evokes, the ideas it gives us, the art and poetry and music it inspired, the stories we told about it and our interaction with it; none of this exists in its familiar form without us: when we are gone, it is gone too (and even the books we leave cease to be books with none to read them).

And that, if you think about it, is the other side of the Jewish teaching (Talmud, Sanhedrin 37a) commonly rendered in English ‘who saves one life, saves the world entire.’ (It is interesting to find that this is the lesson drawn from the tale of Adam’s creation in Genesis, rather than any suggestion that it is some sort of rival to Darwin’s Theory of Evolution: the value of stories is that they teach us how to live in the world).