Cogito ergo sum - I think, therefore I am - was coined by René Descartes in 1637. He was struggling to find a solid philosophical basis for how we know about reality and truth.
This is also turns out to be of the most famous examples of recursion, the process of embedding ideas within ideas that humans seem to do so effortlessly. So effortlessly and so skilfully, in fact, that it's beginning to look like the one true dividing line between animals and humans that may hold up to close scrutiny.
That's the hope of Michael Corballis, professor emeritus of psychology at the University of Auckland, New Zealand. His new book, The Recursive Mind: The origins of human language, thought, and civilization, is a fascinating and well-grounded exposition of the nature and power of recursion.
In its ultra-reasonable way, this is quite a revolutionary book because it attacks key notions about language and thought. Most notably, it disputes the idea, argued especially by linguist Noam Chomsky, that thought is fundamentally linguistic - in other words, you need language before you can have thoughts.
Chomsky's influential theory of universal grammar has been modified considerably since its origins in the 1960s, but it is still supported by many linguists. Its key idea is that the human mind has evolved an innate capacity for language and that all languages share some universal forms, constrained by the way we think. Corballis reckons instead that the thought processes that made language possible were non-linguistic, but had recursive properties to which language adapted: "Where Chomsky views thought through the lens of language, I prefer to view language though the lens of thought." From this, says Corballis, follows a better understanding of how humans actually think - and a very different perspective on language and its evolution.
So how did recursion help ancient humans pull themselves up by their cognitive bootstraps? It allowed us to engage in mental time travel, says Corballis, the recursive operation whereby we recall past episodes into present consciousness and imagine future ones, and sometimes even insert fictions into reality.
We are on our own with this degree of recursion. Chimps, bonobos and orangutans just don't tell stories, paint pictures, write music or make films - there are no great ape equivalents of Hamlet or Inception. Similarly, theory of mind is uniquely highly developed in humans: I may know not only what you are thinking, says Corballis, but also that you know what I am thinking. Most - but not all - language depends on this capability.
If he's right, Corballis's theories also help make sense of apparent anomalies such as linguist and anthropologist Daniel's Everett's work on the Pirahã, an Amazonian people who hit the headlines because of debates over whether their language has any words for colours, and, crucially, numbers. Corballis now thinks that the Pirahã language may not be that unusual, and cites the example of other languages from oral cultures, such as the Iatmul language of New Guinea, which is also said to lack recursion.
The emerging point is that recursion developed in the mind and need not be expressed in a language. But, as Corballis is at pains to point out, although recursion was critical to the evolution of the human mind, it is not one of those "modules" much beloved of evolutionary psychologists, many of which are said to have evolved in the Pleistocene. Nor did it depend on some genetic mutation or the emergence of some new neuron or brain structure. Instead, he suggests it came of progressive increases in short-term memory and capacity for hierarchical organisation - all dependent in turn on incremental increases in brain size.
But as Corballis admits, this brain size increase was especially rapid in the Pleistocene. These incremental changes can lead to sudden more substantial jumps - think water boiling or balloons popping. In mathematics these shifts are called catastrophes. So, notes Corballis, wryly, "we may perhaps conclude that the emergence of the human mind was catastrophic".
Let's hope that's not too prescient.
Else, Liz. 2011. "Thoughts within thoughts make us human". New Scientist. Posted: June 3, 2011. Available online: http://www.newscientist.com/blogs/culturelab/2011/06/thoughts-within-thoughts-make-us-human.html