Kinds of Things

Ana's dog Rex is small, brown and has short legs. Her neighbour's dog Bella is huge and white, with long legs. Rex and Bella look very different - far more different than Rex and the neighbour's brown cat. Yet everyone says Rex and Bella are both dogs, and the cat is not. What makes two things the same kind of thing?

Groups of things

Rex is one thing. "Dog" is not one thing: it is a group that Rex, Bella and millions of others belong to. A group like this, whose members share something, is called a kind. Gold is a kind, and so are tigers, chairs and rain.

Some groups are clearly made up by us. Here is one: the things in Ana's pocket today - a key, a pebble and a sweet. They have nothing in common except where they happen to be. Nobody thinks "things in Ana's pocket" is a real kind out in nature.

Tigers seem different. They were tigers long before anyone named them. Tiger cubs grow up to be tigers. So the question is: are all kinds like the pocket, made up by us, or are some kinds really there in nature, waiting to be found?

Two answers

Some kinds are found. Rex and Bella share something real, even if you can't see it: they come from dogs, and they can have puppies together. Gold is gold because of what it is made of, whatever we call it. The Greek philosopher Plato wrote, about 2,400 years ago, that good sorting cuts nature "at its joints", the way a skilled cook carves meat where the joints are, not just anywhere.

The puzzle for this answer: where is the kind? You can stroke Rex and you can stroke Bella, but you can't stroke "dog". It isn't in Rex, or in Bella, or anywhere else.

All kinds are made. There are only single things - Rex, Bella, this tiger, that one. We sort them into groups because it is useful to us, and different people sort differently. Some languages, for example, use one word for both green and blue.

The puzzle for this answer: if kinds are only our sorting, how can we ever find out that we sorted wrongly? People once called whales fish. Then they learned that whales breathe air and feed their babies milk, and decided that whales are not fish after all. That feels like discovering something about the world, not just changing our minds about a word.

Try it at home

1. Write down five groups you belong to (your family, your class, people who like football, people born in your month...).
2. For each, ask: did people make this group up, or would it be there even if nobody had a word for it?
3. Which ones were hard to decide?

Words to know

• kind - a group of things that share something, such as dogs, gold or rain

Big idea

Some groups, like the things in a pocket, are clearly made up by us. Whether kinds like "dog" or "gold" are found in nature or made by us is an open question, and each answer has a puzzle.