The Wikipedia article on the definition of life states that there is no consensus for the definition of life with at least 123 definitions being proposed.
I am unclear why this is the case.
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I have to say that this question is really why I love learning about biology. Despite all of our progress, there are still mysteries and still disagreements on fundamentals. I suspect that as we build consensus on the fundamental questions such as: what is the definition of life, we will do so only after making further great breakthroughs.
I posed this question because I was told when I was in school (a very long time ago) that the definition of life was the ability to reproduce. This bothered me because it seemed obvious that mules were alive and because it misses, as per my perception, the essence of what it means to be alive.
I thought that there was an obvious definition which I posted in my original question. It is now removed because not only was it naive but it also was not actionable: it had too many ambiguities to be useful as a definition. So, the experience of proposing a definition and failing miserably was very helpful to appreciate the difficulty in building a consensus for any given definition.
I found this article which goes through the 100+ definitions and which discusses in detail why each of the definitions has not succeeded to build a consensus.
Here is a question, at what point in organisation, from molecules to cells, does 'life' suddenly appear?
We can rephrase the question, what is the difference between 'life' and 'non-life'?
The takeaway is, that the definition of 'life', and its distinction between 'non-life', like all definitions (i.e., words and concepts) and distinctions, is based on human whim. In other words, all definitions (i.e., words and concepts) are arbitrary, and are abstractions derived from reality.
Here is another question, if we are constantly replacing the matter, that composes 'us', how do we define or distinguish an individual organism? Furthermore, how do we define an atom, in a non-arbitrary way?
These questions arise, because the distinctions we make between any two things are based on personal choice and because, we do not understand our 'reality' or universe, therefore, we cannot answer these questions.
The Stanford Encyclopedia of Philosophy has an article on Life and gives two broad reasons for why the definition of life has been so hard to agree upon.
First, many people trying to define life are not aware that there are several different types of definitions with different uses. An example the article gives is that NASA's astrobiology lab defines life as, "a self-sustaining chemical system capable of Darwinian evolution". Some people may criticize this definition because it does not include sterile hybrids (e.g. mules, ligers). However, these people do not realize that this is an operational definition, a definition intended to help explain what a project is about. Since NASA's astrobiology lab is only studying reproducing entities, the fact its definition excludes sterile hybrids is a feature, not a bug.
Second, people disagree on is and is not alive, so they build definitions that include and exclude different entities. The original article explains it really well:
Many scientists disagree as to the phenomena a definition of life is intended to unify. Some scientists would include prions, viruses, and entities only hypothesized to exist in the origin of life, while others would completely reject them. Some might accept digital organisms as alive, others would deny this approach.
Because life is just one type of chemical process.
There is a whole spectrum of chemical processes with complex life on one end of it and simple one way one off reactions on the other. Where a humans choose to draw a line and say "life above this line" is very arbitrary. Because of the huge variety of chemical processes it must include many factors, thus it becomes imprecise on a the finest scales because many thing fulfill some and not other parts of the definition. Worse many aspects of a definitions can be vague in and of themselves.
No matter what definition you use you can find things that could be exceptions, gray areas; small viruses that need other cells to replicate, giant viruses that have metabolisms, nucleus-less cells, self replicating polymers, sterile individual organisms.
we can all agree a sterile human is alive, but that means reproduction does not work, but a self stabilizing chemical reaction is not, yet they are both just ongoing chemical processes the only difference is how complex and how many feedback loops they have.
Life and therefore biology is a mess, different branches of biology use whichever definition works best for their purposes. This is actually very common in biology and makes sense when you consider you could be talking about extinct fossil organisms, tiny barely functional microorganisms, huge complex animals, from point of view as divergent as diagnostic medical biology, to evolutionary paleobiology, to extreme condition biochemistry. Just like no one definition of species is going to work for all of these, different definitions of life will work better for some and worse for others.
to put it a different way chemical life will keep finding any way it can to work and it does not care if a few apes can easily wrap their brains around it. this is a common issue in most sciences, humans categories don't always fit nature all that well. Is ice a rock? Is a binary planet a planet? ect.