What's the smallest thing that can be alive?

[Philocrat]

This is the only criterion of yours that is relevant to my post, although it is somewhat misleadingly stated. A more accurate depiction of this stance would be to say "Phenomenal consciousness does not logically supervene on the physical." The claim is not that consciousness is independent of the physical facts, but rather that a complete specification of the physical facts does not entail the existence of phenomenal consciousness. Any purely physical description of the brain is logically consistent with the absence of phenomenal consciousness. Likewise, any functional description of a self-modeling system is logically consistent with the absence of phenomenal consciousness.

Precisely this is what has given me a lot of headache all these years. I keep an open mind, though. If logic makes such separation, there is equally a logical demand to demonstrate without any shaky foundation as to what precisely ignites the union or interaction. Logic may keep them separate as long as it likes...it cannot just hang things there in the logical space without any intention to land. It must subsequently land them on a 'QUANTITATIVELY AND LOGICALLY SOUND GROUNDS'. I for one is waiting curiously for such time!

 

[plover]

There are some distinctions that could be made here.

One is that the Earth (or Earth + Sun, or the Solar system, however one wants to define it; and also ignoring the life-from-comets theories) is the minimal system that we know that is capable of producing life using only non-living material. If you can start with an already living creature, then the simplest creature that can reproduce itself using only it's own internal processes is a bacterium, though even a bacterium, of course, requires appropriate nutrients and a non-lethal environment. In general, a bacterium is the simplest organization of matter that most biologists consider alive, though I don't remember exactly which criteria they use for this.

In the end, in order to define the simplest living thing, you first have to decide on a definition of "alive". So none of the answers given here is necessarily right or wrong—it all depends on the definition being used.

 

[zhana]

No, I actually specifically meant a computer virus when I asked that question. It was something Stephen Hawking said in a lecture that got me thinking about this:

from http://www.hawking.org.uk/lectures/life.html

"A living being usually has two elements: a set of instructions that tell the system how to sustain and reproduce itself, and a mechanism to carry out the instructions. In biology, these two parts are called genes and metabolism. But it is worth emphasising that there need be nothing biological about them. For example, a computer virus is a program that will make copies of itself in the memory of a computer, and will transfer itself to other computers. Thus it fits the definition of a living system, that I have given. Like a biological virus, it is a rather degenerate form,..

Well,certainly it depends on the definition what alive means.
Hawking is too optimistic regarding that definition and no biologist would agree with him that a computer virus is something alive,and many of them would even argue that biological ones are alive (they would tell you a viruses are "on the edge between life and death").And there's a significant difference between these two.A computer virus being immaterial,just a set of instructions.Besides ,remember that alive in the natural evolution means not just a physical form but organic too.

 

[hypnagogue]

Precisely this is what has given me a lot of headache all these years. I keep an open mind, though. If logic makes such separation, there is equally a logical demand to demonstrate without any shaky foundation as to what precisely ignites the union or interaction. Logic may keep them separate as long as it likes...it cannot just hang things there in the logical space without any intention to land. It must subsequently land them on a 'QUANTITATIVELY AND LOGICALLY SOUND GROUNDS'. I for one is waiting curiously for such time!

'Logic' is not the culprit here, but rather, the set of axioms and laws that belong to physics.

In order for any phenomenon P to fit in with any given worldview, one must either give P an axiomatic status within that worldview or be able to derive P from previously existing axioms. Phenomenal consciousness does certainly not enjoy any kind of axiomatic or 'bottom line' status in any physical theory, so we must be able to derive it logically; we must be able to show 'if these axioms hold, then P must follow' (or more shortly, A -> P). But if A ^ ~P is logically consistent, then A -> P cannot be true. And no one has yet been able to provide even a glimmer of theoretical motivation against the proposition that the complete physical description of a human brain is logically, theoretically consistent with the absence of phenomenal consciousness.

In a nutshell, if we want phenomenal consciousness to 'land,' we must provide it with a more suitable theoretical 'perch.' In the meantime, we must recognize the crucial difference between access consciousness (a functional system of global control, 'global workspace,' etc.) and phenomenal consciousness (subjective experience, qualia, etc.). Working within the standard physical paradigm, you could make a case that a system must model itself if it is to achieve the former, but to approach the latter you must provide significantly more theoretical motivation.

 

[Les Sleeth]

No, I actually specifically meant a computer virus when I asked that question. It was something Stephen Hawking said in a lecture that got me thinking about this:

from http://www.hawking.org.uk/lectures/life.html

"A living being usually has two elements: a set of instructions that tell the system how to sustain and reproduce itself, and a mechanism to carry out the instructions. In biology, these two parts are called genes and metabolism. But it is worth emphasising that there need be nothing biological about them. For example, a computer virus is a program that will make copies of itself in the memory of a computer, and will transfer itself to other computers. Thus it fits the definition of a living system, that I have given. Like a biological virus, it is a rather degenerate form, because it contains only instructions or genes, and doesn't have any metabolism of its own. Instead, it reprograms the metabolism of the host computer, or cell. Some people have questioned whether viruses should count as life, because they are parasites, and can not exist independently of their hosts. But then most forms of life, ourselves included, are parasites, in that they feed off and depend for their survival on other forms of life. I think computer viruses should count as life. Maybe it says something about human nature, that the only form of life we have created so far is purely destructive. Talk about creating life in our own image. I shall return to electronic forms of life later on."

I like the definition Maynard Smith offers in his book "The Origins of Life." He suggests that life is minimully that which can participate in natural selection. He points out that a fire metabolizes, crystals reproduce, and both can even be said to adapt to a limited degree . . . but neither are living. Of course "the ability to participate in natural selection" is not a proper definition of life (and it certainly doesn't tell us what "alive" is), but it is useful in the sense of how much must be included to allow the life label by that definition.

In my opinion Hawking is doing what a lot of theorists do, which is to ignore aspects of full living systems that can't be explained so they can call it "alive." If car mechanics were in charge of the definition, then they might define life as a combustion system and claim an automobile is alive. It seems everybody with a metaphysical agenda is trying to spin the facts so their belief system holds water, but if we look at life itself there is nothing fully like it except other life.

 

[Philocrat]

'Logic' is not the culprit here, but rather, the set of axioms and laws that belong to physics.


In a nutshell, if we want phenomenal consciousness to 'land,' we must provide it with a more suitable theoretical 'perch.' In the meantime, we must recognize the crucial difference between access consciousness (a functional system of global control, 'global workspace,' etc.) and phenomenal consciousness (subjective experience, qualia, etc.). Working within the standard physical paradigm, you could make a case that a system must model itself if it is to achieve the former, but to approach the latter you must provide significantly more theoretical motivation.

I think this distinction is well-taken and understood. But as I have pointed it out eslewhere on PF, the former may outlast the latter, for the latter may very well be structurally re-engineered out of place. It may vanish into the former, thus rendering the human conscious existence fully public. And like a goldfish in a glass of water, there will be no place for visual information of all kinds to hide!

 

[Math Is Hard]

I came across this web page, and I thought this person (an oceanographer) had some good things to say on the subject of what constitutes living matter
http://www.oceansonline.com/lifeprops.htm

His key points:
Living matter is organized into complex structures based on organic molecules.
Living matter maintains some type of homeostasis.
Living matter grows and develops.
Living matter reproduces and passes on genetic material as a blueprint for growth and subsequent reproduction
Living matter acquires matter and energy from the external environment and converts it into different forms.
Living matter responds to stimuli from the environment.
Living matter evolves.

 

[Chronos]

How about DNA? DNA = alive, No DNA = not alive.

 

[Gerinski]

Just for discussion, these are the properties listed by Paul Davies in his book "the fifth miracle" for considering something as alive:

- Autonomy
- Reproduction
- Metabolism
- Nutrition
- Complexity
- Organization
- Growth and development
- Information content
- Hardware / Software entanglement
- Permanence and change

He summarizes all of them in just 2 broad items:

1. Metabolism
2. Reproduction

 

[Math Is Hard]

How about DNA? DNA = alive, No DNA = not alive.

Would a computer program that emulates DNA instructions count?

 

[Math Is Hard]

Hardware / Software entanglement

Could you briefly tell me what this means? Thanks.

 

[Math Is Hard]

Something else I find a little troubling - if we consider a brain dead person on life-support, or perhaps a baby that's born without a functioning brain and is kept alive on life support, for a more extreme example... I am not sure if either is categorizable as alive. Or am I back to the parasite scenario again? The living thing is supported by a machine host as well as the environment.

 

[Chronos]

Would a computer program that emulates DNA instructions count?

Only molecular DNA need apply .