simple underlying rules—can generate at least as much complexity as we see in the components of typical living systems.

Yet despite all this, we do not in our everyday experience typically have much difficulty telling living systems from non-living ones. But the reason for this is that all living systems on Earth share an immense number of detailed structural and chemical features—reflecting their long common history of biological evolution.

So what about extraterrestrial life? To be able to recognize this we would need some kind of general definition of life, independent of the details of life on Earth. But just as in the case of intelligence, I believe that no reasonable definition of this kind can actually be given.

Indeed, following the discoveries in this book I have come to the conclusion that almost any general feature that one might think of as characterizing life will actually occur even in many systems with very simple rules. And I have little doubt that all sorts of such systems can be identified both terrestrially and extraterrestrially—and certainly require nothing like the elaborate history of life on Earth to produce.

But most likely we would not consider these systems even close to being real examples of life. And in fact I expect that in the end the only way we would unquestionably view a system as being an example of life is if we found that it shared many specific details with life on Earth—probably down, say, to being made of gelatinous materials and having components analogous to proteins, enzymes, cell membranes and so on—and perhaps even down to being based on specific chemical substances like water, sugars, ATP and DNA.

So what then of extraterrestrial intelligence? To what extent would it have to show the same details as human intelligence—and perhaps even the same kinds of knowledge—for us to recognize it as a valid example of intelligence?

Already just among humans it can in practice be somewhat difficult to recognize intelligence in the absence of shared education and culture. Indeed, in young children it remains almost completely unclear at what stage different aspects of intelligence become active.

And when it comes to other animals things become even more difficult. If one specifically tries to train an animal to solve