The problem, in a word, is language.
Carl Zimmer’s recent article Can Life Be Defined in Three Words raised just that issue.
He referred to the many attempts that have been made by scientists to define what life is, and in doing so, unwittingly exposed some of the main problems in reaching a definition of life.
He described Radu Popa’s study in which Popa went to the trouble of counting the definitions of life, and gave some of the definitions. For example, “Some scientists define life as something capable of metabolism.”

Can you see the problem with that approach?

Those who tackle the definition this way will get nowhere, because they are merely describing a living entity and its functions. They are not describing or defining the life of the entity. This misperception is widespread, and was displayed by several of those who submitted comments to Zimmer’s article.

Zimmer then moved on to the scientist who stimulated the article, Edward Trifonov, who has a more realistic appreciation of what is involved in looking for a definition.

 “Trifonov acknowledges that each definition of life is different, but there’s an underlying similarity to all of them.”  So he set about analysing 150 definitions, and from these, found that what they had in common could be summed up in three words.

 "Life, Trifonov declares, is reproduction with variation."

So close, and yet...

Trifonov is close because he sees that life is not a “thing”, it’s a process. But metabolism is a process. Homeostasis is a process. No good reason was given as to why these are not part of the definition. And unfortunately for Trifonov, it seems he is not getting enthusiastic support for his definition from colleagues.

One, a Nobel Prize winner no less, went so far as to say “Attempts to define life are irrelevant to scientific efforts to understand the origin of life.”

It seems that even Nobel Prize winners can be wrong because this view does not stand up to scrutiny, although it does highlight another problem in defining life, that many who attempt a definition get sidetracked into unnecessary forays into the origins of life.

But surely, although the origins of life are significant and worth exploring, those origins cannot be satisfactorily explained until we have a useful definition of life itself. It’s simply illogical to explore the origin of an unknown.

Trifonov’s tactic of looking for what the many definitions of life have in common was the correct approach, but because he was in effect looking for a consensus view, all he succeeded in doing was to summarise the fallacies of the various attempts.

If, instead of looking for common wisdom, Edward Trifonov had looked f
is; “The possibility that proteins were the basis of life had made them...” (From A Guinea Pigs History of Biology by Jim Endersby. Don’t be fooled by the quirky title; this is a book that everyone with an interest in the subject should have on the shelf!)

We see statements like these in newspapers, magazines, text-books, and on the television almost daily. But they are nonsense. Proteins and related compounds are not the fundamental units of life, or the basis of life, or the building blocks of life. They are the building blocks of living entities as we know them on this planet.

Life and entity are completely different concepts.

Because this flawed language is so frequent and pervasive, it has conditioned people to the point where they cannot properly discuss the matter.

They cannot distinguish between life and entity. (Again, see the comments following Carl Zimmer’s article)

Yet it’s such a simple distinction.

When we ask “What is life?” we are in effect asking “What is the essence of a living organism; what distinguishes it from a non-living entity?”

We would not proceed to describe enzymes or proteins or DNA, as these are also applicable to a piece of steak. Similarly, if we were asked to define the essence of an internal combustion engine for example, we would only describe the parts of which it is comprised as background information.

We would not explain the essence of the engine until we described the conversion of energy and its transfer.

In other words; the essence of the internal processes of the engine.

The same situation exists in defining life.

Trifonov needed to go one step deeper, to find the essence of the processes that make up a living system. Those processes are reproduction, metabolism and homeostasis, and they have the common base that Trifonov was looking for.

All he needed to do was look here - What is Life? (
Not only would he have found a practical, universal definition of life that is applicable to cells, organisms, communities and alien life if any exists, he would have also found to his delight that the definition has an additional feature of absolutely no importance at all – it is comprised of only three words.