In other words, general set theory is pretty trivial stuff really, but, if you want to be a mathematician, you need some and here it is; read it, absorb it, and forget it.
The only way to learn mathematics is to do mathematics.
The best way to learn is to do; the worst way to teach is to talk.
The only way to learn mathematics is to do mathematics. That tenet is the foundation of the do-it-yourself, Socratic, or Texas method,...
The beginner should not be discouraged if he finds he does not have the prerequisites for reading the prerequisites.
Feller was an ebullient man, who would rather be wrong than undecided.