A fundamental mystery underlies this season's Republican presidential primary debates: How can it be that the two highest-polling contenders -- Donald Trump and Ben Carson -- are also two of the field's least capable debaters? In the Republican candidates' fourth appearance together, Trump and Carson once again showed themselves to be out of their depth. Operating in a milieu that calls for command of the issues, both hid behind their familiar, personality-based shtick, leaving the heavy lifting of intellectual engagement to their rivals onstage. The theme of this Fox Business Channel debate was the economy, which ought to have handed Trump an automatic advantage. Yet instead of using the platform to highlight his strong suit, Trump frittered away his time.