Exit Gary Cohn. Enter a stock market bogeyman: The unknown

What always worried Wall Street about President Trump was his unpredictability. Despite being one of the best-known faces on television and having opinions well-publicized by media new and old, he lacked a legislative resume that investors could use to assess the strength of his commitment to campaign-trail promises.

The antidote to their anxiety was Trump’s early selection of Gary Cohn, a 57-year-old investment banking executive, to fill the post of National Economic Council director. So, it wasn’t a surprise when Cohn’s resignation brought the fear factor back.