In some notes on derived stacks, in describing categories of fibrant objects, the author drops this parenthetical:
(Grothendieck said in his famous letter to Quillen that the choice of $\mathscr F$ is like the choice of a basis of a module.)
Question 1: Where can I find the quote that this refers to? What is the wording of this quote?
Question 2: Can someone explain this analogy? My understanding of model category related stuff isn't good enough for me to understand this.