Cohen, a biologist and Stewart, a mathematician, are science fiction fans who have made a hobby of doing the science for a number of leading SF writers. In this book they combine science and science fiction to challenge the view, expressed in Ward and Brownlees Rare Earth, that complex life will be a very scarce phenomena They argue that Ward and Brownlee's arguments arc too parochial and have been in part falsified by the discovery of life in extreme environments here on earth.
They think the same goes for much standard astrobiology, which presumes that life must be like life on earth, or rather what we imagine life on earth to be like in oversimplified accounts. They propose a much more radical approach of xenoscience and xenobiology. This 'life' may be much more alien than anything we can imagine (nuclear life on neutron stars, or plasma life in the Sun's photosphere).
One thing we can be certain of is that no real aliens will be bipedal humanoids or anything else related to terrestrial organisms. They suggest some features which have evolved many times over on earth might be more universal (flight of some kind for example) This doesn't mean this will be the only kind of life out there, . merely that it might reoccur in quite a number of different places.
What most writers call intelligence Cohen and Stewart call extelligence, a shorthand for . cultural intelligence mediated by complex 'language' capable of transmitting highly abstract 'ideas' and information, and of preserving this outside the bodies of particular individuals. They seem to be willing to entertain the idea that this extelligence may be widespread and perhaps a universal development, but have to concede that of all the millions (or perhaps billions) of species that have evolved on earth, only one, ours has produced this extelligence. It has to be faced that only one terrestrial culture went through an industrial revolution and developed heavy industry of the sort needed to build radio telescopes and spaceships.
Even without this argument Cohen and Stewart reject ufologists images of the ETH. If ETs are present in our environment it will not be as biological entities, but as some form of 'technology' so subtle that we could never detect. The aliens of ufology, like those of Star Trek, etc. are essentially cultural icons, secularised forms of the creatures of folklore and myth.
As to whether Cohen and Stewart, or Ward and Brownlee are right, who knows at this juncture. If we encounter the truly alien in the oceans of Europa then put your bets on Cohen and Stewart, if all we find is nothing or just terrestrial type organisms courtesy of some passing meteorite. then go for Ward and Brownlee. But forget the Greys and their breeding programme (of course if we are wrong and the Greys actually exist then we know that they aren't ETs at all, but some other terrestrial creature we have somehow overlooked perhaps descended from some as yet undiscovered branch of the hominid bush) -- Peter Rogerson