In Act Three of The Crucible, Mary Warren comes to the courts and attempts to explain that the accusations that the girls were making were false, and were based on fear and mass hysteria. It started with wanting to get out of trouble for dancing and conjuring spells, but continued because of fear of reprisal from Abigail, and because of the power of mass hysteria. Mass hysteria is where everyone around you exhibits a particular emotion, and so you feel yourself experiencing that emotion too. For example, if you are in a crowded movie theater watching a horror movie, and someone screams in fear at a part that isn't particularly scary, you might feel fear and scream yourself--just because everyone else is screaming. Mary tries to explain this happening in the courts when someone was accused. She says to the judges,
"I--I heard the other girls screaming, and you, Your Honor, you seemed to believe them, and I--It were only sport in teh beginning, sir, but then the whole world cried spirits, spirits, and I--I promise you, Mr. Danforth, I only thought I saw them but I did not."
What Mary is describing is mass hysteria--everyone was crying out and screaming they felt spirits, so she too felt it. Her logical mind realized that they weren't there, but the emotion was contagious.
Danforth, Parris, and Hawthorne decide that if she was able to fake it in court, then she should be able to fake it right then and there. They hypothesize, "let her turn herself cold now, let her pretend she is attacked now, let her faint." Mary tries, but can't--since all of the other girls aren't screaming, and because she is under intense pressure, she can't do it. And, because of that, the judges assume she is lying about it all, and that there really were spirits. Her attempt to tell the truth falls flat, simply because the judges think that if she can't faint alone, then she must not be under the spell of a spirit at that time.