Late last week when Cathleen Black resigned after just three months as New York City’s schools Chancellor, many influential New Yorkers decided that the way to analyze the event was as evidence of their mayor’s stumbling. That’s quite a turn of interpretation for a man who, for several years, seemed to be able to do no wrong. Not that long ago, Michael Bloomberg was being talked about widely as a post-partisan presidential possibility. The fact that the bloom is now off the rose, and constituents are said to be tiring of him, offers an important lesson in power for all of us.