Colin Tudge considers how symbolic thought sets man apart from the animals

Rene Descartes suggested in the 17th century that thought depends upon language and since animals have no language comparable with ours, he concluded that they cannot think. QED.

Generations of animal lovers since have known in their bones that animals do work things out and are wont to bewail: “If only Rex Joey Tiddles could speak!” Professor Herb Terrace of Columbia University, New York, declared at a Royal Society symposium on animal intelligence in 1984: “Of course animals think. Our task is to find out how they do it without language. ” Then there is the small matter of consciousness. Even if animals think, do they know that they are thinking? Do they know that they exist at all?

Terrence Deacon has provided what seems to me to be good working answers to these ancient questions and a great deal else besides. Even the lowliest creatures “think” up to a point. But human beings have evolved an extra trick which none of the others have, and it makes all the difference.

To behave as they observably do, animals cannot simply operate like a telephone exchange as the behaviourists hypothesised. They must create some internal representation of the world around them, and then manipulate this representation. This is “thought”.

But Deacon identifies three levels of representation of increasing complexity, sophistication, and usefulness; and the difference between them is critical. At the lowest level is the icon, simply a reflection of the outside event. More complex is the index, compounded of icons, which represents correlations between different events that may be separated by space andor time. At the highest level is the symbol - a kind of meta-sign, which actually signifies other symbols and the relationship between them. The icon and the index both refer more or less directly to the world outside, but a creature that can symbolise can create an entire, literally imagined universe inside its own head, which need be referred to the world outside only for the purposes of checking.

The brain that can symbolise can rehearse the future, planning entire scenarios in minute detail, bringing to bear its symbolised memories of the past. For a brain that can represent the world symbolically the universe is its oyster.

The one Earthly creature that has evolved the skill to symbolise is, of course, ourselves. The symbols are the raw materials of human language. In ecology, as in war, the tiniest tricks make the difference between overwhelming success and abject failure. William I is supposed to have won at Hastings primarily because his horsemen had stirrups: the side that had the technical innovation, albeit an unspectacular one, obliterated the other. The internal manipulation of symbols, as opposed to cumbersome icons, is just a mental trick, though far from trivial. Thus equipped, Homo sapiens has emerged as a new kind of creature, unprecedented on this Earth, that has absolutely dominated all the rest.

With this simple notion, much else seems to fall into place. Descartes was wrong to suggest that animals do not think. They clearly do, as Deacon says; and we need not suppose that they can refer only to what is immediately around them, or are unable to recall the past andor contemplate the future. But Descartes was right to identify a qualitative difference between human and animal thought. Language is indeed the key. We represent the world to ourselves in symbols while the others must work with less versatile imagery, like scene shifters.

Modern evolutionary psychologists suggest, too, that our brains have been shaped like the rest of us by natural selection, and that our genes predispose us to particular predilections and ideas - a notion that some have chosen to misunderstand and take exception to.

Well, genes certainly underpin all life on Earth but as Christopher Badcock of the London School of Economics is wont to remark, “Genes make brains”; and when brains start to play with symbols, arbitrary and abstract representations of the world, they build dreams from them which become reality. Our brains have indeed evolved by natural selection and are definitely an asset, but because they have evolved the skills they have they can lead us into very weird territory.

Several fine and illuminating books on the working of the brain, including Marvin Minsky’s The Society of Mind and John Tooby and Leda Cosmides’s The Adapted Mind, have appeared since John Maynard Smith remarked in the 1970s that this was the great unsolved mystery of biology. Terrence Deacon’s book takes us another step forward. We’re getting there.

Colin Tudge is a research fellow of the Centre for Philosophy at the London School of Economics