The definition of life

How do you define life? What characteristics would an extraterrestrial entity need to be defined as living?

DNA?

traditional definitions of life involve self-replication and reaction to stimuli. i think DNA is NOT a necessary condition for life as it is theoretically possible an alternate chemistry could perform the encoding function differently.

Selfreplication
Adaptation to medium / hability to adapt the medium to itself
Evolution to progress in getting better on previous points.

Reproduction, Respiration.

Easy, innit.

Why respiration? How do you define respiration? Does a bacteria respirate?

yes they do. respiration is the process of converting sugars into ATP/whatever else energy source. so even plants respirate, after they photosynthesize.

EDIT: as defined, there are probably other chemical energy sources that would not count as respiration so i say that's not a good requirement.

Why does the process have to be chemical? Isn't it possible for an organism (theoretically, at least) to get energy from, say, atomic nuclear reactions?

Is a virus alive? A Prion?

Ideas replicate, are they alive?

A virus cannot replicate without a host. Of course, many organisms are dependant on other organisms to survive. I think that it would be good to try to pin down exactly what we mean by replication and respiration - or any other definitions of life. Being specific helps.

Is it is considerated aislated, no because they couln't selfreplicate itself, but if you define the enviroment of virus as a life cell, then yes, because virus can selfreplicate on a cell. So virus are life inside life. Remove the host and you kill the virus.


Using the same argument, no outside of human, but yes if you restrict ideas to that enviroment then yes.

OF course, a lot of people restrict the meaning of life only to selfcontained matter with this properties and anothers like methabolism, so the only life we know is carbon life on Earth.
But another people extend that and they don't have problems naming ideas "life" in this sense.

I think that life can only be defined relative to the life we know. The only way you can define anything is according to what you know. So, be the definition general or specific, it has to be an extension of the life we know of. Otherwise, the definition doesn't have any meaning. Or rather, it has a made-up meaning.

Better hope it has DNA I'll probably shoot it if it dosen't !

Monroe

Sorry bout that little buggy
Not!

Hey, don't shoot the mRNA.

I'd like to think of life as an opposing force to decay. Non-living things just are, and wither away over time. Living things keep repairing and recreating themselves. In more physical terms, perhaps we could say that something is alive if it comprises one or more processes that locally decrease entropy?