Christians sometimes hear the following rebuttal for Christ's resurrection: the disciples may have been well-intentioned, but in their grief and sorrow they suffered from individual hallucinations which led to group hallucinations. Here are two comments from respected scholars on that theory:

Biblical and historical scholar N. T. Wright writes,

Recently some people have tried to say, "Ah, well, when those you love die, sometimes you will experience them in the room with you, smiling at you, maybe even talking to you; and then they will disappear again. Maybe that's what happened to these disciples."

[That] is a well-documented phenomenon as part of the grief process … but the crunch is that the early Christians knew about phenomena like that as well. They knew perfectly well about such things as visions, hallucinations, dreams, ghosts, and so on. In other words, if they'd had an experience, however vivid it seemed, of being with Jesus, but if the tomb had not been empty, they would have said, "My goodness, this was very powerful, and quite consoling in a way; but he hasn't been raised from the dead of course, because dead people don't get raised (until all the dead are raised at the end)—and anyway, there is his body in the tomb."

Dr. Gary A. Sibcy, a licensed clinical psychologist writes,

I have surveyed the professional literature (peer-reviewed journal articles and books) written by psychologists, psychiatrists, and other relevant healthcare professionals during the past two decades and have yet to find a single documented case of a group hallucination, that is, an event for which more than one person purportedly shared in a visual or other sensory perception where there was clearly no external referent.