Mr. Hale has suggested that Deputy Governor Danforth allow John Proctor and Giles Corey to leave the court and come back with a lawyer to present their argument that the girls are lying, based on the new testimony provided by Mary Warren.  In response, Danforth claims that a lawyer would be of no real help here because the lawyer would not be able to identify a witch.  In fact, Danforth cannot even identify a witch, unless she were to attack him directly.  He says that, since witchcraft is an invisible crime, only her victim would be able to identify her since the witch would never accuse herself.  This kind of testimony, where a witch's victim might swear that a witch sent out her spirit to attack the victim, is referred to as spectral evidence (the witch's spirit is her "specter"), and this kind of evidence was permissible during the Salem Witch Trials—in fact, most people were convicted on spectral evidence.  Because of the nature of spectral evidence, only the accuser could provide it, and it would be her word against the accused witch's.

Danforth says that the best person to identify the witch is his/her victim.  He continues that since witchcraft is an invisible crime, only the victim and the perpetrator know for sure a crime has been committed.  What this really means to the accused is that there is really no way for them to effectively defend themselves agains an accusation since it is only their word against their accusers.  This also means that it is an open field for would be accusers to accuse enemies for any reason.

Danforth says a witch's victim can best identify a witch.

Act III, page 93:

DANFORTH: "...But witchcraft is ipso facto, on it's face and by its nature, an invisible crime, is it not? Therefore, who may possibly be witness to it? The witch and the victim. None other. Now we cannot hope the witch will accuse herself; granted? Therefore, we must rely upon her victims..."