Read more: 鈥�Special issue: What is reality?鈥�
PHILOSOPHERS are not being rude when they describe the approach most of us take as naive realism. After all, when they cross the street on the way to work, they tend to accept implicitly 鈥� as we all do 鈥� that there is an external reality that exists independently of our observations of it. But at work, they have to ask: if there is, how can we know?
In other words, the question 鈥渨hat exists?鈥� reduces, for what in philosophy passes for practical purposes, to questions such as 鈥渨hat do we mean by 鈥榢now鈥�?鈥�
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Plato had a go at it 2400 years ago, defining 鈥渒nowledge鈥� as 鈥渏ustified true belief鈥�. But testing the justification or the truth of beliefs traces back to our perceptions, and we know these can deceive us.
Two millennia later, Ren茅 Descartes decided to work out what he was sure he knew. Legend has it that he climbed into a large stove to do so in warmth and solitude. He emerged declaring that the only thing he knew was that there was something that was doubting everything.
The logical conclusion of Descartes鈥檚 doubt is solipsism, the conviction that one鈥檚 own consciousness is all there is. It鈥檚 an idea that is difficult to refute.
鈥淚T IS DIFFICULT TO REFUTE THE IDEA THAT CONSCIOUSNESS IS ALL THERE IS鈥�
Samuel Johnson鈥檚 notoriously bluff riposte to the questioning of the reality of objects 鈥� 鈥淚 refute it thus!鈥�, kicking a stone 鈥� holds no philosophical water. As Descartes pointed out a century earlier, it is impossible to know we are not dreaming.
Nor has anyone had much luck making sense of dualism 鈥� the idea that mind and matter are distinct. One response is that there is only matter, making the mind an illusion that arises from neurons doing their thing. The opposite position is 鈥減anpsychism鈥�, which attributes mental properties to all matter. As the astrophysicist Arthur Eddington expressed it in 1928: 鈥渢he stuff of the world is mind-stuff鈥� not altogether foreign to the feelings in our consciousness鈥�.
Quite separately, rigorous logicians such as Harvard鈥檚 Willard Van Orman Quine abandoned the search for a foundation of reality and took 鈥渃oherentist鈥� positions. Let go of the notion of a pyramid of knowledge, they argued: think instead of a raft built out of our beliefs, a seaweedy web of statements about perceptions and statements about statements, not 鈥済rounded鈥� in anything but hanging together and solid enough to set sail upon. Or even, possibly, to be a universe.
This idea is circular, and it鈥檚 cheating, say critics of a more foundationist bent. It leads back to the suspicion that there actually is no reality independent of our observations. But if there is 鈥� how can we know?
