If we see a dog running, or a cow chewing cud, or a man
shouting loudly on the street, we know that these are living beings. What
if the dog or the cow or the man were asleep? We would still think that
they were alive, but how did we know that? We see them breathing, and
we know that they are alive. What about plants? How do we know that
they are alive? We see them green, some of us will say. But what about
plants that have leaves of colours other than green? They grow over
time, so we know that they are alive, some will say. In other words, we
tend to think of some sort of movement, either growth-related or not, as
common evidence for being alive. But a plant that is not visibly growing is
still alive, and some animals can breathe without visible movement. So
using visible movement as the defining characteristic of life is not enough.