The question of what it takes for something to be "alive" is an old one. While the difference between "alive" and "not alive" seems obvious, it turns out to be extremely difficult to actually define the difference.
For quite a long time, the predecessors of scientists thought they had this figured out. Obviously, living things are possessed of some factor which non-living things don't have. Just what it is was pretty mysterious, but there was obviously something. These "natural philosophers" called it the vital spark, or the vital principle.
The idea was that anything that was alive was possessed of one of these vital sparks, while things that weren't alive...weren't.
So what is a vital spark? Well, the description was that you couldn't see it, taste it, feel it, hear it, or smell it. You couldn't capture it in a bottle or any other vessel; you couldn't transfer it from one object to another. You couldn't measure it or detect it. In other words, the only way to determine if an object had one was to determine if it was alive or not.
This isn't actually a very useful idea, since it pretty much devolves to exactly the same problem as the "what is the definition of life" question does. Defining the vital spark is the same as defining life.
Obviously, this concept is a supernatural one, and we have already discussed the fact that modern scientists don't accept the supernatural as a reasonable explanation or answer.
The concept of vitalism led to some lively debates among scientists of particularly the 19th century. It was central to the arguments over whether life could pop into existence by spontaneous generation. It was also part of a debate in chemistry over whether the organic chemicals which are produced by living things could be synthesized in the laboratory; the strict vitalist was sure that they couldn't.
He was wrong. Over the lost century and a half, essentially all substances which can be found in living things have been manufactured in vitro (literally, "in glass"). But the search for whatever the "vital spark" was never found an answer.
Today's scientists do not believe in the existence of a vital spark. The biologists of the late 20th century are "mechanists." This isn't a very graceful term, and it suggests a lot of mistaken ideas, but essentially it means that biologists don't believe that there are any substances or materials which are exclusive to living things. What makes something alive is not what it's made of; it's how it's put together and what activities (i.e, chemistry) go on within its structures.
Go to your text for a discussion of the features which are accepted as descriptive of "life." You will look in vain for a "definition." There doesn't seem to be one.
Oh — and there also doesn't seem to be a very sharp line between "living" and "non-living." There's a grey area in between which houses some things which are sort of alive, but not quite. The major inhabitants of this murky region are the viruses, which exhibit several of the features which we generally consider the exclusive property of life — heredity, evolution, reproduction — but are completely lacking some of the other essential aspects of life — metabolism, cell structure, homeostasis. So are they alive, or aren't they?
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