The Flame That Passes Six Tests

Light a candle and look at the flame properly, the way you would look at an animal you had never seen before.

It moves — it leans, it climbs, it flickers. It takes in fuel — the wax goes down all evening. It takes in oxygen — cover it with a glass and it dies. It gives off waste — soot, and a gas that would turn lime water milky, exactly as in lesson 22. It grows — a bigger wick gives a bigger flame. It responds — blow on it and it bends away from you.

That is six of the eight things a biologist checks for. Six out of eight, and a candle flame is not alive, and everybody knows it is not alive.

So something is wrong with how we are using the list.

Here is what is wrong. The tests are not equal, and the two that feel most obviously alive are the two that decide the least. "It moves" and "it grows" are what almost every person names first — and a river moves and grows, a flame moves and grows, a crystal grows. Those two tests, on their own, tell you almost nothing.

The two that do the work are quieter, and neither is about being busy:

Can it make more of its own kind? And is it made of cells?

A flame cannot and is not. Neither can a river, a car, or a crystal. A cat can and is. A mango tree can and is.

One warning before we start, and it is not a technicality. Even the encyclopaedias say outright that defining life is somewhat contested, and they name the virus as the case that breaks the rules. So this lesson will teach you a checklist that works, and then show you the two things it cannot settle. That is not the lesson going wrong. That is what the edge of a real subject looks like.

By the end of this lesson you'll be able to:

• list the characteristics of life and use them on something you have never seen
• explain why a candle flame passes most of them and is still not living
• say which characteristics actually separate living from non-living, and why
• explain why a dry seed is alive even though it does almost nothing
• say why the virus is argued about, without pretending the argument is settled