Define life

On the basic level: anything that uses things found in its environment to produce copies of itself.

 

life is from the moment i wake up til i go to bed

 

I like that one.

 

A shot at it.

The criteria in this order:

1. Stimulus response
2. Metabolism (ability to process things from the environment)
3. Homeostasis
4. Reproduction

I define life as something that posesses at least the first 3 characteristics and is the offspring of something with all characteristics or can produce offspring with at least the first 3.

 

"Reproduce" as in "cell reproduction" rather than species propagation.

But yes, I agree that it isn't a proper definition.

Excrete as in release waste products from the system.

Sorry it seems I should have elaborated. ("system" means organism in question)
- Feed: ingest chemicals into the system
- Breathe: take in oxygen and give out carbon dioxide
- Respire: convert chemicals they feed on to release energy
- Excrete: release waste products from the system
- Grow: increase in size
- Reproduce: multiply on a cellular level, or on the system level (i.e. make babies)
- Respond: ~ to external stimuli
- Move: umm... move.

Homeostatis? How do plants do that? I thought this was a characteristic of animals, rather than life in general. Plants exchange gas at different rates depending on the light level, and overproduce CO2 or O2 as a result. That would indicate that they don't care much for keeping their levels of various chemicals constant.

 

Mise's definition seems pretty good to me... what living thing doesn't exhibit those traits?

 

For instance... An individual worker ant/termite/bee doesn't reproduce. Plants don't move. Not all beings grow throught their life. Certain butterflies never feed after they emerged from the cocoon.

 

So what. You're gettin examples of specialization... and in the case of butterflies, it's not because it doesn't want to feed anymore that it hasn't feed itself before to grow.... so it's rather pointless.

But anyway, I like a lot the definition from DP. I like especially its simplicity.

 

Ant's etc. are made up of living cells which reproduce. Also, it could be argued that the *species* is capable of reproduction, therefore members of the species are living.

Plants move! Their flowers open, their leaves fold at night, etc.

What beings don't grow throughout their lives?

Those butterflies still feed though, whilst in their cocoons. Humans feed, even though they dont do so for most of the day.

 

I think it says it all. Who would deny that something which takes energy from its environment and uses it to copy itself is alive? Nobody serious I think.

 

Fire does that too

Damn fire! It spoils every theory!

But it's one thing to say, "all living things do this," but quite another to say "no non-living things do this which living things do." Man that was a confusing sentence!

 

Not so fast. By that definition, thunderstorms would be alive. They take energy from the environment and make little thunderstorms. And What about waves in the ocean?

Edit: x-post with Mise.

 

I've seen this idea talked about here. It may well be correct to state that non-life to life exists along a spectrum and there is no real clear dividing line between the two ends.

 

What if we only know of a single member (a worker and) of an ant species. The species itself has no chance to reproduce (through usual means). Do we consider that ant living?

Then we can only argue those living cells are alive and not the ant itself. The definition for a species requires a DNA. If we meet something on an alien planet which exhibits certain characteristics (stimumus-response, homeostasis,...) and we don't find a structure symilar to DNA can we conclude we have found life?

Again what I said to Marla. If we have found a single butterfly that does not feed and that can't be catalogued in any known species, can we conclude it is life?

Plants don't "move about", but I'll agree if you originally meant this.

I'm trying to find a set of necessary and sufficient criteria for life. Meaning, given those and only those we can say for sure we have found life and not life in the general sense (species), but individual. In other words, what tests can we put a small/large lump of "something" to qualify as life.

 

Hmm.., Fascinating... I must think now.

 

Best of luck, sincerely. When you do find it do not forget to send it to NASA. They need it too.

Btw, that is a key question in Astrobiology. I mean, how can we test for something if we cannot even define it properly?

 

I can try, can't I?

Give a shot at my criteria, and don't be afraid to tear them appart (as I know you will).

 

No, that's neither true in the case of thunderstorms nor in the case of waves since in both cases they are reproducing smaller entities of themselves... Not copies of themselves. That makes a huge difference.

Moreover, I should add that in our instinctive way to apprehend what life is. The fact it's an autonomous being is extremely important. A plant grows in a specific way to reach sunlights. Neither storms nor waves do such a thing. They aren't looking for energy, they don't specifically go where there are energy. They have absolutely no freedom of movement. Just like the example of the Earth.

When you want to get counter-example, you're forced to distort it.


Edit : Actually, in the case of storms, waves or fire... those phenomenons are simply energy. Life isn't energy, Life uses the energy in its environment to reproduce copy of itself.

 

Well, fire releases energy, but in its most familiar form, it is atoms of carbon combining with atoms of oxygen to produce that energy. Fire is not simply energy, it has a rather physical presence that quickly disperses in the process.

The point that the threadstarter may have wished to raise here is that life is an emergent property of basic physical processes, that life becomes more so as it evolves more complexity.

That idea has been much more aptly expressed in the link I provided above.

 

So what.
How does that prove my point is wrong ?