Even the best minds were not always perfect (at some point, he thought positron could as well be thought as an electron going backward in time ---which we today

Even the best minds were not always perfect (at some point, he thought positron could as well be thought as an electron going backward in time ---which we today know isn't the case). This is a quite similar story: ghosts have the wrong sign for the kinetic term: http://en.wikipedia.org/wiki/Faddeev%E2%80%93Popov_ghost http://en.wikipedia.org/wiki/Faddeev%E2%80%93Popov_ghost Note that he of course didn't try to introduce negative probabilities as basic things. In both cases it was just a matter of interpretation --just with "bad" physical consequences.