Definitions

Philosophers often use "necessary and sufficient conditions" to define words or concepts. Is quality X necessary in order to be a Y? Is it quality Z sufficient to make this thing a Y?

Try it on something familiar, say, a bat. "A bat is an animal that flies." Is flying necessary if it is to be defined as a bat? Yes. Is it enough? Not really, when there are also fish and birds that fly. We need some additional quality - maybe "A bat is a mammal that flies"? Would that do?

What about a car? "A car has an engine and four wheels." Are four wheels necessary? No, there are 3-wheeled cars around. Are four wheels and an engine sufficient?  No, something with four wheels and an engine could be a train engine or a tractor or a bus... Can you do better?

Necessary and sufficient conditions can be very useful, but don't work very well for everything that we might want to define. Try them on "art" or "games" or "person", and you'll see the difficulty. We may have to find some other way to define our terms.