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Comment by tbabb | Hacker News Reader

To me, there seems to be a pretty straightforward line to draw between life and non-life: Life is that which undergoes Darwinian evolution.

If (a) it makes copies of itself, (b) the copies inherit traits from the original, (c) the traits are subject to variation/mutation, and (d) the success of the copies at making further copies depends on those traits which are inherited, then evolution must occur.

Every other definition seems to me over-complex and ad-hoc; like specific things we have noticed which tend to be true about most life on Earth, but which might not be true for special cases, or for some other hypothetical extraterrestrial life which we might discover someday.

This definition is an unambiguous binary classifier, and it seems to correctly categorize everything we would intuitively call "alive". (Also, under this definition, viruses are unambiguously life).

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typically i'd ignore semantic arguments but since this is in reference to philosophical consideration i'll assume you are someone who actually enjoys getting into the weeds of semantics.. if i am wrong in that assumption please feel free to ignore the superfluous following

your definition loses your purported binary classification when examined through rigor

> (a) it makes copies of itself

what about those forms of life that are unable to procreate, or choose to abstain? also 'copies' is ambiguous: see (b);

> (b) the copies inherit traits from the original

seems an unecessary condition.. if one is to accept the first than this is redundant, also it is confusing because you say 'inherit traits' and 'copies', a copy should inherit all traits, so which are we talking about? copies or trait inheriting progeny? if 'copies' is intentional what about forms of life unable to make exact copies of itself, like humanity?

> (c) the traits are subject to variation/mutation

if 'variation/mutation' is random one possibility would need to be completely foregoing the 'variation/mutation'

if 'variation/mutation' is deterministic then are there deterministic models of 'variation/mutation' that you would leave out of your definition for life?

(d) the success of the copies at making further copies depends on those traits which are inherited, then evolution must occur

what about life that fails in its own ability to continue to evolve but heavily affords other life to evolve further.. i'm thinking something like wheat that is scientifically controlled in its evolution because we forced its evolution to a sweet spot that for years helped humanity to evolve as we wanted

Right - your objection under A includes mules, as an example. (Normally they're sterile.)

surely we must include mules in life.

I think they could be included by allowing for evolutionary dead ends. That is, if mutations are random, not all will be successful long term (though pecularities in their specific environment may enable their survival for some time).

You are missing a ridiculous large space of trait possibilities. But then, nobody ever remembers that.

Always test your definition against fire and crystals.

Fire: Does not mutate.

Crystals: Do not mutate, or if by some twisting of definition you say they do (e.g. defects are propagated through the lattice), the success of propagation doesn't depend on the inherited traits.

Hmm. If Darwinian evolution is based on factors currently present around the species, wouldn't it basically be optimizing for short-term factors? So you could call Darwinian evolution a greedy algorithm, that optimizes for a local maxima. What about the global maxima, how can life evolve to that state?

If I understand your question, the answer might be something to do with what happened in humans; we developed brains large enough to be able to recognise patterns in our own behaviour and abstract those across all aspects of life.

Having said that, the long term stability of that path is definitely debatable!

> What about the global maxima, how can life evolve to that state?

It doesn't. Evolution is indeed a greedy algorithm, and cannot plan.

As long as we're ok with the fact that by this definition, it's easy to make artificial life and we already have. (Take the entities from any sort of evolution simulator.) I guess you would call that "digital life", and contrast it with "physical life".

EDIT: I'm confused about the downvotes. To be really concrete, the swimmers in Darwin Pond[1] seem to me to satisfy all of OP's criteria for life. Do they not?

[1] http://www.ventrella.com/Darwin/darwin.html

I would say digital patterns which follow the rules above could be called "life", albeit artificial and exceedingly simple compared to biological life.

Is a computer virus that mutates itself via an evolutionary algorithm considered life?

I would say yes. It's just not biological

What? That's like saying "this drug causes sleep, it's just not soporific".

What makes you think life can only be biological? What about artificial life and digital organisms?

The word biological just refers to life or living organisms. So anything that's alive is also biological, by definition.

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