"Life" is one of those words that challenges us to "define" it.
On the one hand, life on Earth is an observable phenomenon. It includes ourselves, and it includes many other types of life, which all share the following characteristics:
There are some very specific details about how all this happens which are shared by all known life forms on Earth:
Given these shared and very specific detailed characteristics of all known life forms, the inevitable conclusion is that all known Earthly life forms are descended from a single common ancestor.
The fact of common origin throws us a problem when we want to define "life". Although we have many different examples of living things, in one sense we have only one example of life, which is the set of living things that happens to be descended from this one single common ancestor.
What is "life"? Is it the set of Earthly life forms, all descended from one universal ancestor? Or is it the set of possible things that could exist, at least hypothetically, within a Universe that has the same physical laws as the one we live in, where those things are similar in some essential aspects to the life forms that we know about?
Potentially we could search for more examples of life, perhaps in space, living on other planets that circle around other stars, or perhaps in the laboratory when some scientists can create some artificial "life" which is distinct from all natural life forms.
But these alternative examples have not been found yet, so they remain hypothetical.
When we attempt to "define" life, what we are really doing is guessing about what the universe is capable of.
There's not necessarily anything wrong with guessing, as long as we don't pretend to ourselves that we are doing something else.
There are many observed features of life that might be included in a "definition" of what life is.
One of things about life that distinguishes "alive" from "dead" is the dynamic nature of life.
Living things don't just sit there, they do stuff.
But there are many system on Earth, or in the Universe, which are dynamic in some way, but which we do not want to include in our definition of life.
One category of dynamic things is found out in space. Gravitationally bound objects circle around each other, and they continue to do so for very long periods of time.
These systems are dynamic, in one sense, but the persistence of movement in these systems is very dependent on the absence of dissipation. For example, the Moon will go around the Earth in its orbit, for a very long time, because there is nothing much slowing it down, or speeding it up. (Currently the Moon's orbit is speeding up, because tidal forces cause it to absorb angular momentum from the spin of the Earth. But that won't last forever, and eventually other things will alter the relationship between the Earth and the Moon, in particular the expected expansion of the Sun into a red giant in a few billion years.)
On Planet Earth, all dynamic systems are subject to friction of one kind or another, so all dynamic systems that exist are dependent on external sources of energy. Which is mostly (but not entirely), the energy generated by the Sun.
I want to start my attempt at defining life with a requirement that life be both dynamic and subject to dissipation.
Then I want to discover just one or two more abstract principles which distinguish life from all those other dynamic phenomena which are subject to dissipation.
All known Earthly life is descended from a single universal common ancestor that existed long, long ago.
One consequence of this historical fact is that, apart from events that happened at the time when life originated, any life only exists as the result of the continuous prior existence of life.
If we don't restrict our consideration to dynamic systems, then this condition also applies to static systems. For example, the existence of a particular rock at some particular time depends on the continuous prior existence of that same rock at earlier times. (If we go back far enough, we will observe that the rock came into being as the result of dynamic geological processes, but that could have been a very, very long time ago.)
Similarly, this condition also applies to dynamic systems that are not subject to dissipation (or at least it applies to binary systems of gravitationally bound objects in space – systems of three or more objects are typically not dynamically stable over the long term).
We can extend the requirement for dissipation to a requirement for the necessity of dissipation, if we consider "dissipation" as the same thing as the occurrence of thermodynamically irreversible processes.
We observe that Earthly life forms are not only subject to thermodynamically irreversible processes, but that they depend on thermodynamically irreversible processes to ensure their own continued existence.
So I propose a set of four conditions as a definition of "Life":
The fourth condition does subsume the first two conditions, because if the continued existence of life depends on thermodynamically irreversible processes, then life is necessarily dynamic and subject to dissipative forces.
However, as I have stated above, attempts to "define" life are really just guesswork about the capabilities of the Universe, and given the tentative nature of the guesswork that I am engaging in here, I prefer to retain the first two conditions as separate conditions, just in case it turns out that the fourth condition isn't really needed.
What is abstract about my proposed definition is that it doesn't mention RNA, DNA or Protein.
What is very abstract about my definition is that it also doesn't mention homeostasis, reproduction or evolution.
