I like the approach Dawkins takes, at least I think it's him, of looking at life as a unique occurence (to our knowledge) - a total system with a single origin. It doesn't make sense to have a definition of life/unlife based on the characteristics we observe (reproduction etc.) as we only have one instance of life to go on. If there were more than one instance of life-like systems (i.e. extraterrestrial life) then we could make a more sensible classification.

Hence prions, viruses, etc. are part of that system, and therefore do count as 'life'. Prions, and mobile genetic elements (the simplest virus-like systems) are part of that living system, even if not independent organisms. But all but the very simplest and oldest of organisms can only exist as part of a system of life - we're no more independent from the totality of the biological system than prions are.

My thoughts anyway, I'd be interested to have someone take these apart :)

Life only has value in that it is a means to an end. Of course they are life, but that question is irrelevant. The question is do they have value to me. And they have tremendous value, for things like research purposes.

Mmm yes and for spraying indiscriminately over small, oil-rich countries. And Tunis. Damn Tunis.

@hopsy - interesting take, i like it, biologists probably wouldn't, but hey they like their (often stupid, and historically biased) definitions which are about as valid as the gaia hypothesis.

Gaia, by the way, is not a useful description of the planet, if only because we don't have any other life-planets to compare it to, so again being unique it is 'life' by your definition, but unless that idea provides and useful insights into how this 'life' operates (which you would expect to get by comparing it to other kinds of life) but we don't know about any other kinds of life-planet - other bits of the biosphere are just reductionist approaches to describing the world biosphere, enviroment and global climate. (the life-planet attempts to regulate it's own temperature - from the external effects of varying solar output - because mechanisms which manage to keep the temperature in the same range are the kinds of mechanisms which will keep themselves around - natural selection?

delicate balance? ok i think i've said enough for today... interesting.