From Richard Findlay
David Prichard mentions seven criteria for recognising a life form: assimilation, excretion, growth, locomotion (movement), reaction to stimulus (irritability), reproduction and respiration (7 July, p 21). I would suggest all you need to do is travel to another planet and start a fire. Fire exhibits all seven vital functions, and accordingly a fire on Mars would count as an alien life form.
Of course in most cases an oxygenated atmosphere would be necessary for fire to survive and procreate, as I observe at work as a firefighter. But the same could be said for many species on Earth.
Aberdeen, UK
