Several economists argue that central bankers should consider asset prices along with consumer prices in their inflation targets. The problem with this suggestion is that nobody can prove definitively that share prices are rising because of speculative pressure rather than a genuinely improved outlook. As Alan Greenspan asked in December 1996, “how do we know when irrational exuberance has unduly escalated asset values?” Only in retrospect does the answer to this question become clear.

According to the text:

Item 2 - For Alan Greenspan, it is impossible to tell whether markets are irrationally exuberant.