Aristotle defined them:
1. Invention: finding ways to persuade.
2. Arrangement: putting together the structure of a coherent argument.
3. Style: presenting the argument to stir the emotions.
4. Memory: speaking without having to prepare or memorize a speech.
5. Delivery: making effective use of voice and gesture.
One could make a strong case for the first three being all you need to know about producing anything written.
The fourth, "Memory," is important in order to be able to summon up appropriate examples and analogies at a moment's notice without having to stop and look something up.
This can interrupt an otherwise easy flow of words, thoughts and ideas.