The quote from Arthur Miller above refers to the characters and actions of the characters as portrayed in his play The Crucible. Given that the hysteria which broke out during the Salem witch trials was simply based upon the grudges and lack of morality of the characters, and the real people involved in the trials, the fact that individual conscious could have kept the trials from happening in the first place is very important.
In the play, once the characters realized how easy it was to accuse another, all they had to do was say a name. Once the name was stated, all eyes were turned on that character. The character, regardless of truth behind the accusations, immediately became suspected of witchcraft.
Therefore, if a person were to rely on their conscious, they would not have made accusations against others based upon hatred, grudges, or desire for more wealth. Basically, what Miller is stating, is that the lack of conscious is what fueled the numerous accusations which flew through Salem.