There is a philosophical truth that no one can prove a negative; yet, journalists expect candidates to do this all the time. Case in point: the airwaves are all atwitter with allegations that Herman Cain had engaged in some kind of "sexually suggestive behavior" to at least two female employees when he was the head of the National Restaurant Association in the 1990s.
Note the vagueness of the accusations, as reported by Fox News:
The report said the women signed agreements with the restaurant group that gave them five-figure financial payouts to leave the association and barred them from discussing their departures. Neither woman was identified.The report was based on anonymous sources and, in one case, what [the Politico website] said was a review of documentation that described the allegations and the resolution.