I am unclear about what it means to be "alive." Is fire alive? What are the criteria that fire satisfies to make it so? Or, what criterion for being alive does it violate?

The definition of life is a fuzzy one.
I think that in practice, life is defined by it's members, rather than by any precise criteria (ie whether something is alive or not is arrived at by consensus rather than directly by objective analysis).

Is fire alive?
It consumes chemical energy from raw materials (eats)...
It consumes oxygen (breathes)...
It reproduces...
It dies...

The last two are a bit suspect, however.
Does fire really reproduce? Does a "child" inherit properties of the "parent"?
What does dead fire look like?

A dead fire looks like carbon.

That would be a partly burnt stick. It was never fire; it is the remains of the fire's fuel(food).

Ash is to fire as excrement is to animals.

I thought that water was needed for "life"... but I can't choose if I will say "yes" or "no" to this interesting question!

The Seven Traits of Life

Sensitivity and reactivity to the environment and Capacity for adaption
Ingestion of substance for energy to function
Reproduction
Respiration
Emission of wastes
Internal Movement
Cellular Structure


Fire is not alive.
Fire is a chemical reaction, a rapid oxidization, that happens to make invisible gases glow with thermal energy transmitted by conduction and convection.

It's important to remember that there's no fundamental difference between living and non-living matter. Whether or not we choose to categorize something as 'alive' is completely arbitrary.

Have you been watching backdraft Errandir??

No fire isn't alive, because 'life' has a free will (in most people's definition).

So plants have free will now?

LOL

Here you say that sensation or awareness (of sensation) is a trait of life.
Your following quote denounces the fact that a mere physical-process can be equated to life:-
So, how do you reconcile the two statements? On the one hand you say that a physical process is not life. And in the second hand, most here would say that physical processes are the cause of life.
If this is the case, then life is a mere physical-process, which means that there might be life in fire.

No Pete, that's why I said 'life' has a free will (in most people's definition). I would say most people do not class plants as *living, hence a pre-definition for live is free will.

*Edit: Sorry, I didn't mean non-living.

Mucker, you're not making sense.

Plants have no free will.
Plants are alive.
Therefore, having free will is not a necessary condition for being alive.

Where's the problem?

You might be able to make a good argument for fire being alive. At the most basic level, living things like plants and animals are just self-sustaining chemical reactions, much like fire. Of course the chemical reactions that allow plants and animals to exist are far more complicated then the relatively simple reactions involved in combustion, but they are both reducible to self-perpetuating reactions.

This is why fire shares so many of the characteristics of life commonly accepted in biology; fire reacts to the environment, ingests substances for energy to function, reproduces, respirates, emits wastes, and has internal movement. The 'cellular structure' requirement was more or less tacked on arbitrarily so that we wouldn't have to consider fire to be alive.

Well what exactly is fire? If we're just talking about the flame then that's nothing more than superheated gas. If we talk about the chemical reaction, that's just a process which is taking place within the molecules of another object, be it wood, or hair or whatever.

Mucker, there's no such thing as 'free' will. Everything is random chance, or determined outcome, affected by many factors, from chemical reactions, gravity, and quantum menchanics. We have no choices, just possibilities.

I was about to say : "Cellular Structure! That's the solution. Thus fire is not alive!", but I thought about AI. If we agree that "Cellular Structure" (biological) is life, then we say that a robot with an AI (in 10000 years) will not be alive. Here, I disagree!

Thus, the problem is still open for me.

Perhaps if we can somehow prove evolution to be true, this would imply that fire is not alive, because it never evolves.

Does everything evolve? (bacteria)

while it is generally agreed that fire isn't alive, this is one of the most common examples of why our simplification of "what is life?" isn't the most accurate. the second most common example is that Viruses and priors aren't technically alive by the formal definition, though they certainly seem to act like they are alive.

The Seven Traits of Life

Sensitivity and reactivity to the environment and Capacity for adaption
Fire reacts to changes in it's environment. it moves when wind blows, it changes color when it's fuel is changed. it burns in spheres in zero gravity.

Ingestion of substance for energy to function
Fire externally (sort of) ingests fuel, and uses it to survive. whent he fuel runs out, the fire 'starves' and goes away.

Reproduction
fire can move from one fuel source to another via wind or heat transfer, you end up with two flames instead of one.

Respiration
fire uses oxygen in it's reactions, and therefore respires.

Emission of wastes
fire gives off water and ash as it's products in most cases.

Internal Movement
ever watch a fire burn? it certainly has internal movement.

Cellular Structure
nope.


The main rub seems to be that fire doesn't have a cell membrane of some kind. But then again, neither do viruses or prions, really. there isn't a diffinitive boarder which seperates the fire from it's surroundings. it is really not a thing, but a different state of it's surrounding medium (hotter air/gas than the non-flame near it).

However, if you look close enough at cell membranes, they are just a high consentration of their surrounding material. lipid bi-layers will slef assemble in solution, but a beaker with proto-cell membranes floating in it isn't alive. Look even closer, to the atomic level, and most of everything is empty space, so where is the clear dividing line between what is inside the cell and what is out? there really isn't one.