To understand the furor over the decision by Standard & Poor’s, the rating agency, to downgrade U.S. government debt, you have to hold in your mind two seemingly (but not actually) contradictory ideas. The first is that America is indeed no longer the stable, reliable country it once was. The second is that S.& P. itself has even lower credibility; it’s the last place anyone should turn for judgments about our nation’s prospects.

Let’s start with S.& P.’s lack of credibility. If there’s a single word that best describes the rating agency’s decision to downgrade America, it’s chutzpah — traditionally defined by the example of the young man who kills his parents, then pleads for mercy because he’s an orphan.

America’s large budget deficit is, after all, primarily the result of the economic slump that followed the 2008 financial crisis. And S.& P., along with its sister rating agencies, played a major role in causing that crisis, by giving AAA ratings to mortgage-backed assets that have since turned into toxic waste.

Nor did the bad judgment stop there. Notoriously, S.& P. gave Lehman Brothers, whose collapse triggered a global panic, an A rating right up to the month of its demise. And how did the rating agency react after this A-rated firm went bankrupt? By issuing a report denying that it had done anything wrong.