I would rather go with “life is a figment of imagination of someone who is creatively challenged”.
Q: What is a life-force?
A: Something that enables an entity to live
Q: What is living?
A: Hanging around till it self-replicates
Q: Like a computer virus?
A: No, a computer virus is not a physical entity. Only a physical entity can have life
Q: Then, if I design a robot that can self-replicate, does it have life?
A: No. Robot is made of tin sheets and metal/plastic parts. It doesn’t have life.
Q: Then who can have life?
A: Only entities made up of organic compounds can have life
Q: Then if I make a self-replicating robot using plastic, does it have life?
A: No, only an organic entity using DNA as a self-replication mechanism can have life
Q: What if we find an alien that uses SNA, instead of DNA, as a self-replication mechanism? Will it have life?
A: I guess so. But we don’t even know that aliens exist
Q: Humor me, will you. We are just doing a thought experiment. But my alien friend is made up of inorganic material. Like Transformers. Or like the cute little Coke-can aliens that come in the movie Batteries Not Included (a must see). I guess you won’t treat them like machines. Will you?
A: They have feelings. So, I’ll treat them like living things.
Q: Bacteria don’t have feelings. Don’t they have life?
A: Of course, they are living things.
Q: Then why is my robot not a living thing?
A: Because they are man-made. Living things are natural. They are made by God.
Q: Well, if God is responsible for all my actions, can’t I assume that God used me as a tool to get the robot made? In other words, God made the robot, but used me as a tool to achieve his end.
A: Well, I am not in a mood to go on cooking up any more silly answers.
You see, there are no water-proof definition for life. Any which way we try to define life, there is always one condition it which the definition fails. Still, we know a life form the instant we see it. How we do that is we simply recognize certain characteristics that are common among DNA based self-replicating entities on earth. Like the way they are shaped, the way they feel for touch, they way move, the way the morph, the way they replicate and so on. That’s all.
To make matter worse, because we couldn’t understand the complexity of the construction of the living things, we ended up attributing a fictitious force/agent (we seem to have a compulsion to imagine the presence of an agent even when nothing is there). We assume that the agent called life-force gets into the life forms and makes it do stuff, be stuff, etc.
Of course, all agents must have a purpose. And who gives the purpose for life? A super-agent of course. The mother, or father, of all agents. The God! So, the only definition of life that holds water is this: A meta-physical agent, who derives its power and purpose from the God, that drives an entity.
For me, that is way too convoluted. I would rather go with “life is a figment of imagination of someone who is creatively challenged”.