Recent advances in scientific research claim to create "artificial life". They are only replacing DNA in living cells. I cannot find research that describes what life is, where it comes from, how it permeates inanimate molecules and makes them "live". Putting aside the impossible complexity of living cells required to capture and retain life, where does life come from in the first place? We've discovered dark energy and dark matter as being necessary to maintain the state of the universe, yet we can't detect them. We have no idea what gravity is, but it may originate in alternate dimensions. Is it plausible to consider life to be an energy that exists as dark energy exists? Is it all around us and only manifests itself within the proper matrix? Would it exist even if nothing was "alive" in the universe? What is it?

What is the difference between a living thing and a non-living thing? What is "vitality"? This is a difficult question. Once upon a time, it was widely believed that living things are distinguished by possessing a certain substance (an "elan vital") or perhaps by a certain force being present in them alone. This was a legitimate, testable scientific theory ("vitalism") that now appears to be false, since living processes can take place outside of living things (as when digestive enzymes can break down food in the test tube).

Another notable family of views on this question is that living things are alive in virtue of the fact that they carry out certain "life functions" such as growth, self-motion, metabolism, reproduction, and so forth. This view would account for the intermediate cases between life and non-life (such as viruses and whatever entities existed in the early stages of the origins of life on Earth). The intermediate cases could presumably carry out some but not all of the life functions.

Even if there were some sort of substance or energy that is possessed by all and only living things, presumably what makes that substance or energy able to give life is that it allows the living things to perform these characteristic life functions. That's another plus for the life-functions view.

However, it is notoriously difficult to specify these life functions in a way that all and only the canonical living things perform them. I set as a challenge for you to think of things that are not alive but can reproduce, respond to their environment, and undergo evolution by natural selection.

Another interesting complication here arises from the notion of "artificial life" -- not just life created by tinkering with DNA in living cells, but bits of software that behave like living things (such as computer viruses). Bits of computer code can reproduce, respond to their environment, undergo evolution by natural selection, and so forth. Are they alive (or even intermediate cases like biological viruses)? This is a controversial question. Some people think that if a bit of computer code simulates living things closely enough, it goes beyond merely simulating them to being actually alive. Other people think that this is an absurd result and shows that the life-functions view is mistaken.

Another option here is to say that what sets living things apart is not merely that they perform (many of) these life functions, but rather how they do so. Perhaps living things carry out these life functions "emergently": by bottom-up processes the precise outcomes of which cannot be predicted by some calculational short-cut. Some famous cellular automata are good examples of emergence (though they are not alive and perform none of the life functions).

There are many interesting examples in the history of science where the fact that something was alive was invoked to account for its behavior in much the way that an object's being copper can explain its properties and behavior. Whether these are genuine scientific explanations or not will ultimately depend on whether vitality turns out to have an essence or not. This remains an open scientific question.

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