I was reading Halmos Naive Set Theory, Chapter 2 The Axiom of Specification, in which after stating the Russel Paradox, he goes on to say "The moral is that it is impossible, especially in mathematics, TO GET SOMETHING FOR NOTHING. To specify a set it is not only enough to pronounce some magic words; it is also necessary also to have at hand a set to whose elements the magic words apply."
This is after he shows that a universal set cannot exist ( in the naive perspective of set theory of which this book is about).
What is bothering me, is the statement GET SOMETHING FOR NOTHING. I cannot seem to understand what he means there. If anyone has read the book and could explain it it would be a great help .