Now Tristram wants to talk some nonsense (as if he hasn't all along).
A little background: Momus was the Greek god of mockery and blame. According to Lucian, Momus wanted humans to be created with a window that would look into their souls, so everybody could see exactly what they were thinking and feeling.
Yeah, let's all be grateful that advice was ignored.
Tristram thinks that would have had two ridiculous consequences. First, people would have had to pay tax on their windows (windows were taxed in England until 1851). Second, writing a man's character would have been too easy. All you'd have to do is peep in the window and write down exactly what you saw. Instead, writers have to approach character indirectly.
Tristram's preferred method is to talk about his uncle's hobby-horse. Aha, the pieces are coming together.