Topics Biology

Question Why is an amoeba considered alive, but a car is not? The car is as complicated as the amoeba. It eats gasoline, and produces waste. It also has a reproductive system: by providing humans a useful service, cars have been able to use human factories as breeding grounds. When a car stops working, we say that it dies. Finally, if you look at cars through the decades, you can see that the primitive species of car (i.e. Model T) evolved into modern species (i.e. Prius). So why aren't cars alive?

Accepted:
September 24, 2009

Comments

As you suggest, there are a lot of analogies between cars and living things, and if you had used computer programs, there might have been even more. So, if we wish to say that amoeba and such are alive, whereas human artifacts are not, we need to find the relevant differences. There are at least three salient ones that seem relevant to picking out what counts as living (and the proper subject of biology):

1. What they do: Self-replication. All living things can replicate themselves. No artifacts can. Of course, it gets tricky when you consider things like computer viruses. Or future robots that might build robots like them. People also talk about other functions such as metabolism and self-regulation, but they might offer even less clear boundaries between living and non-living.

2. Where they came from: Evolution from a common ancestor. That is, the current (well-supported) theory is that all living things share a common ancestor. No artifacts evolved from living things.

3. What they are made of: Living things are made of organic material, and (related to points 1 and 2), all have RNA or DNA (the ancestral material that allows self-replication). No living things are made of these materials.

Having picked out these three criteria to distinguish living from non-living things, a good philosopher might ask, as you have, why these criteria are important. She might ask whether the complexity or functional properties shared by both a car and a horse or by both a sophisticated robot and a human are more interesting and important than the criteria above. But there is no obvious reason why the words "living" or "life" need to be used to pick out these interesting similarities, nor do these words prevent useful comparisons between living and non-living things.