It's something worth thinking about because that "something" seems obvious for straightforward examples like rocks and humans. But it is much less obvious for entities like viruses or viroids or even prions. Along those very murky lines, it is not obvious to anyone what constitutes life and what doesn't. Scientists draw up very precise definitions to include or exclude certain entities from life. And not all of them agree. There is no sharp dividing line and where a dividing line is drawn, it is done with definite measurable criteria. It's clear from your answers that even though you claim a sharp dividing line, you don't really know what that line is. You're in good company!
In thinking about abiogenesis, the same murky life-non life distinction exists. Afterall, nobody is suggesting that humans sprang from rocks (accepting some creationists). The gulf between life and non life is certainly not as clear-cut as you proclaim and it is possibly not a very big one.
Actually, Nebullis, I can propose a basis for the "sharp dividing line," but it's not physical; it's informational, Shannon information. Living beings are such because they are at the receiving end of a successful communication that reduces uncertainty in the receiver. When they are no longer successfully communicating, life ends. This is being discussed at Plato
so I won't duplicate that effort here.
Shannon does not at all focus on the meaning (semiotic content) of the communication, nor does he stipulate its source (the sender). We can speculate about that. My thought is that the communication is directed to the organism to "tell it how" to modify its internal boundary conditions so as to store and harness thermodynamic entropy to biologically useful outcomes, rather than to entropic "heat death," effectively reversing, or at least postponing the inexorable slide towards the equilibrium that the second law predicts for physical systems. I think the communication may be carried/facilitated by a universal vacuum field, or as an astrophysicist friend has suggested, an "organic zero point field;" possibly the transmission of the communication is facilitated by particle (electron, photon) exchanges with the ZPF. It has been further suggested that DNA is a sort of "router" that enables the source of the communication to send it to the correct "address." It seems the information or communication source may not be located within the 4D block of familiar spacetime, but proceeds from an extra-dimensional source which may be geometrical. Clearly Einstein thought that at its ultimate basis, the universe is the expression of a geometry. So did Pythagoras and Plato, as it turns out.
I know it all sounds so sci-fi; but (not counting the hypothetical extra-dimensional/geometric basis, which he has not explicitly considered thus far), in addition to providing a fascinating description of the nature of life, the result of a wide-ranging collaboration with a number of first-rank thinkers in a broad range of scientific disciplines (you would readily recognize their names), particularly in the fields of thermodynamics and information theory, he's gotten it all boiled down to equations by now. I can't say much more about it, unfortunately, for the manuscript is a work-in-progress, in preparation for publication. But maybe you'd like to read the book when it comes out.
But I am really off-topic with this here. So please, anyone wishing to reply to this post, please do so on the Plato thread?