How to understand the quote "beginner should not be discouraged if he finds he does not have the prerequisites for reading the prerequisites"

Question

It's a quote from Paul Halmos, he said that "The beginner should not be discouraged if he finds he does not have the prerequisites for reading the prerequisites."

What is the idea he tried to convey? Is it a reminder for the book authors to write self-contained book? Or is it an encouragement for beginners?

Answer 1

Tertuliano Franco has one of the best interpretations for this quote I have seen, in the back cover of his soon to be released book on the Principles of Combinatorics and Probability. Here a snapshot

The translation is more or less:

That is, studying mathematics is often a non-linear activity and prerequisites should not be taken literally.