It’s alive! Power over life and death with aru and iru

Aru and iru are two of the first words that students of Japanese learn. Aru is for inanimate things and iru is for animate things.
テーブルの上にペンがある。tēburu no ue ni pen ga aru There is a pen on the table.
庭にワニがいる。niwa ni wani ga iru There is a crocodile in the garden.
The pen is inanimate, so we use aru. The crocodile is animate, so we use iru.
Aru can be used with people, and in old Japanese there only was aru for everything. Even today it is perfectly acceptable grammatically to say:
あの人は子供がある。ano hito wa kodomo ga aru That person has children.
However, this is just ticking the ‘has child’ box, rather than conjure up images of living breathing children.　In this case, iru is preferable, and as a non-native speaker if you use aru it may sound like a mistake. The distinction between iru and aru gets more interesting, though, when you consider what it means to be ‘animate’.

Live fish for sale in a supermarket, would use aru. They may be alive, and animate, but they’re food, first and foremost. Live bait is the same.

スーパーに活魚がある
sūpā ni katsugyo ga aru
There are live fish (for sale) at the supermarket.

ワニの生き餌として使うウサギがこのかごの中にある
wani no ikiesa toshite tsukau usage ga kono kago no naka ni aru
The rabbits that we'll use for the crocodile's live bait are in this cage.

Lice and fleas as well can be distinguished in this way. Iru and you’re talking about a creature, aru and you’re talking about an infestation. The use of aru depersonalises them; they’re condemned.

肩の上にノミが一匹いる
kata no ue ni nomi ga ippki iru
There is a flea on (your) shoulder.

このハリネズミはノミがある
kono harinezumi wa nomi ga aru
This hedgehog has fleas.

On the other hand, you can use iru to give life to inanimate objects.

急いでいるときにかぎって、タクシーがいない。
isoideiru toki ni kagitte, takushī ga inai
There's never a taxi when you're in a hurry.

The taxi moves about under it’s own power, so you use iru. They move around freely and independently. Trains don’t get the same treatment, though. You can’t use iru with a train, because they’re stuck to a track and a timetable. There’s no free will. In fact, just having a person associated with the car can warrant the use of iru. If you see a police car outside your house:

パトカーがいる patokā ga iru

Because you know that there’s a policeman nearby, possibly in your house and asking too many questions. But if it was parked outside a police station

パトカーがある patokā ga aru

It’s just a car.

Cuddly toys in a shop for sale would be aru. Who cares about them?
But a child’s favourite toy would be iru, because of the personal relationship between the toy and the child. In Toy story, Andy would use iru with Woody, even without seeing him move. Because to Andy, Woody is alive! In Toy Story 3, he might use aru, though. Poor Woody.

ウッディとバズがベッドの上にいる Uddi to Bazu ga beddo no ue ni iru Woody and Buzz are on the bed.

ウッディとバズがゴミ箱の中にある Uddi to Bazu ga gomibako no naka ni aru Woody and Buzz are in the bin.

The distinction is very much at the discretion of the speaker, and given the right context, almost anything could use either.

NHK has a remit to provide television that commercial channels don’t, and produce quite astounding concepts such as panel shows about washers. I don’t mean anything related to cleaning, but the small disk that goes on a bolt to help the nut stay on. As I say, NHK take their remit very seriously. The fact I was watching such a programme says more about the quality of commercial television in Japan than it does about my interest in washers. Anyway, after various explanations about the importance of washers, they wheeled on a rusty old bicycle. Do you think we can unscrew this bolt? They asked, showing just how rusted and manky the nut was. A quick spin of the spanner, and off it came. The big reveal: there was a washer there, stopping the nut from rusting to the bolt. In surprise, one of the panellists exclaimed: ワッシャーがいた！Wasshā ga ita! This wasn’t just a simple ring of metal: it was the hero of the day. By using iru, she’d granted the washer a soul.