Everyone knows you’re not supposed to bet on a bubble, but what about a potential bubble?

As the S&P 500 Index climbed above 5,000 on Friday, many of us found ourselves wondering what exactly we should call this market: a bubble or a durable new bull? Are we reliving 1995 — when, coincidentally, a previous “soft landing” economy pushed the Dow Jones Industrial Average above 5,000 en route to a 25% annualized return over the rest of the decade — or 1999, when the by-then-flimsy stock market edifice was on the verge of collapse? As is too often the case, it’s hard to know.