1. Poirot and Miss Marple had to be genii, because their creator was a genius.
2. Each time, a crime is committed; each time, everyone is a suspect, and everyone has a perfect alibi.
3. With second-class writers, there would often be failings in the plot.
4. With Agatha Christie, there are hardly any.
5. There is just one vital clue that only the detective, and the very astute reader, can pick up.