The word “life” has probably been around ever since mankind began using language. It is a word of fundamental importance to all of us, and seldom do we make it through an entire day without putting it to use. We do so, however, with only a sketchy and subjective idea of what life actually means. This is because until recently, within the last century or so, it has been easy for people to distinguish between what they call living and what they call non-living. There has been no need to define life precisely; its meaning is intuitively understood. Life’s properties clearly distinguish it from everything else. Every living thing is a cell. Every cell is bounded by its own outer membrane and contains a full set of instructions necessary for its operation and reproduction. Furthermore, every cell uses the same operating system. There are hundreds of billions of different proteins used by living things, but all of them are made from the same twenty amino acids, the “building blocks of life.”
Living things reproduce themselves. Either individually or in sexual pairs, they have both the encoded instructions and the machinery necessary for self-reproduction. Some creatures cannot reproduce, but every creature comes from reproduction. Periodic crystals like sodium chloride (table salt) also undergo a kind of self-reproduction. In crystals however, the “instructions” are much simpler, they are not encoded, and they are not different from the “machinery.”
Life uses processes collectively called metabolism to convert materials and energy for its needs. Metabolism creates waste products. When metabolism ceases with no prospect of starting again, we call it death. Machines also convert materials and energy for their needs, create waste, and could be said to die.
Life undergoes evolution. Notably, simpler forms are succeeded by forms with greater organisation. Cars evolve also, in their way. Computers do, too. And computers even contain their own encoded instruction sets.
These latter properties of life are sometimes used to make the point that life is hard to define. But nothing else has all of these latter properties except cellular life using life’s DNA—RNA—protein operating system. Another kind of life, entirely different from ours, is conceivable. But the only kind we have ever seen is the one we are part of here on Earth.
Viruses and prions are not alive; they lie on the fringe of life. Viruses contain instructions encoded in DNA or RNA. (Prions don’t.) Both are reproduced. Viruses certainly and prions probably can evolve. But neither can reproduce itself; each needs the machinery of a living cell to carry out its reproduction. Without a cell, viruses and prions are merely inert complicated particles which do nothing. Do they make it hard to define life? No, just as trailers don’t make it hard to define motor vehicle traffic. We know what motor vehicle traffic is. And we know what life is.