Very largely we have indeed defined a specific set of physical properties and capacities that will distinguish life from non-life.
The problems have only arisen at the lower fringe, when we try to classify, first, viruses, and then prions. It may be that the nearer we get to the mud, the harder it is to draw clear lines.
The really interesting problems will arise in AI. My first draft of an answer there says that what makes us biological entities 'alive' includes the way our brains operate very largely by responding to situations and questions with feelings before proceeding to seek solutions.
That is, our capacities to reason are encased within emotions, which determine such basic things as WHAT we want to do or solve, and WHY we want to do or solve it, and WHETHER it's urgent and WHETHER it justifies particular levels of expense ─ and so on.
Without emotions, we wouldn't get out of bed in the morning or any later time, because we would have no motive to do so, and the fact that immobility would lead to stress and pain and ultimately cessation of function would be irrelevant from our PoV ─ because without feelings, everything is irrelevant.
And while we can get our robots to simulate particular emotional responses (as robot dogs already do and no doubt sex dolls will), it seems to me the moral question doesn't arise for us unless and until we see something authentically similar to our emotions in our bots. And when the moral question arises, so do the questions about whether they're 'alive' in a meaningful sense ─ or so it seems to me in this first draft.
Will we get to the point that androids judge us, assume that we don't have emotions (at least not like their emotions), then eliminate us?
When we experiment with artificial intelligence, and "almost" get it right, do we scrap an android with a nearly perfect set of emotions because we want to try again? Would we be creating and destroying life? Murder?
If we developed androids that don't want to work, but just replicate and sit around on couches watching football games (hmm....we seem to have succeeded in duplicating human emotion), do we continue to pay for their housing and maintenance or do we scrap them?
Maybe this is the joy and frustration of God?