You can't discuss what is 'unnatural' unless you first establish what 'nature' is. Few words in the English language are more flexible than this one--especially since the eighteenth century made it a buzzword.
Percy has done this discussion a service by noting the plurality of meanings. The two definitions he provides definitely have to be taken into account in a discussion like this.
Neither are those two meanings exhaustive.
In popular usage 'natural' often means 'that which is achieved by the most economic and elegant means.' The opposite of 'natural' in this case is 'contrived.' (It takes its cue from the 'natural/artificial' dichotomy but this one can refer entirely to the work of human beings:
The speech writer sought language that sounded natural.)
A teleological use of the word 'natural' is based on the 'form follows function' principle. It means 'as nature intended' or 'as God intended.' In this usage, it is natural to use your mouth to eat. You need need to eat in order to survive; of all your anatomical parts your mouth is uniquely qualified for the job of eating. It is less obvious, and thus 'less natural' by this definition, to use your mouth to breathe, talk, sing, kiss, practice oral sex, or pull the cap off a bottle of ale. If the speaker finds any of these 'less natural' uses personally revolting, he or she may move the slider over into the red zone and call it 'unnatural.'
A widespread Industrial Age romanticism toward nature has brought us the popular idea too that 'natural' means 'beneficial.' This connotation is assumed, profitably, by the purveyors of health and health food products. To speak of 'natural ingredients' is to speak automatically, it is thought, of ingredients that are good for you.
But gas gangrene is natural. Blood flukes are natural. Tapeworms are natural. The idea of 'natural' as 'beneficial' is thus not... well, natural.
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Edited by Archer Opterix, : trying for more natural prose.
Archer
All species are transitional.
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