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Comment by klez | Hacker News Reader

Sure, but considering how central and defining the concept of "life" is to biology (the study of life and living organisms) you'd think we wouldn't have a fuzzy definition for that specific concept. I can see why it's tricky, though.

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Life is very useful as a term because it allows you to define a 'not living' term as well: dead. And it has meaning at the highest level. But if you start looking at things in a more detailed way even death doesn't arrive 'all at once' for multi cellular organisms, for instance a dead person's hair still grows to the point that corpses need to be shaved. And a virus may be dead by one persons view on what 'life' is all about but alive by someone else's definition. And depending on the context both of them may be right.

The definition is fuzzy because the concept is fuzzy! Even something that we in every day life see as settled such as a species is not always clear-cut. Cat or dog? That's usually easy. Member of a species yes or no? Not so easy, and in some cases subject to considerable debate and even then unresolved.

For instance:

https://en.wikipedia.org/wiki/Species_complex

And:

https://en.wikipedia.org/wiki/Ring_species

Boundaries are hard, just like naming things.

Agree on the fuzzy term.

It is misconception that hair and nails continue to grow. What happens is the that kind and soft tissues dehydrate and shrink and the hairs and whiskers stick out more. Growth stops soon after oxygen and nutrients stop being delivered.

Ah good one! Another childhood myth killed off.

We really only have one example of life (or at least all our examples are interconnected), so I don't expect great definitions.

Just like geology doesn't have a great definition for their subject of study (the earth). They have a definition that works really well, but because they only have one example, the definition ain't stress tested.

Slightly less silly: it took the discovery of lots more bodies inside and outside the solar system (dwarf planets here, exoplanets elsewhere) for astronomers to really nail down the definition of planet.

I find Maturana’s definition satisfactory. Best overview in his book with Francesco Valera:

Autopoiesis and Cognition: The Realization of the Living (1980)

https://en.wikipedia.org/wiki/Autopoiesis_and_Cognition:_The...

Unfortunately out of print.

how does it exclude fire, something well known for making more of itself (which seems to be the main criteria in Autopoiesis)

I think that depends on your prior expectations about how biological concepts should be structured. I was trying to make the case that we should expect that they're fuzzy when we're dealing with very complex phenomena that exhibit a lot of variation. The fact that these kinds of phenomena happen to exhibit clustering is what makes (fuzzy) classification possible, but we also find that many phenomena or organisms are in borderland areas between clusters, so the classification doesn't work as well with them.

The problem is that it's really hard to come up with a definition that includes all of the things we agree are obviously life (e.g. mold), does not include fire, and does not just appeal to the particular structure that most or all life on earth seems to have (the cell).

The result is a landscape of fuzzy definitions mostly centered around that last one.

We only have a single tree-of-life (or possibly, several syncretic trees-of-life from a single planet) as an example. Makes it a little difficult to discern the true principles.

Analyze a word you think has a crystal clear definition.

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