In answer to Hale, Danforth puts forth the argument that witchcraft is an "invisible crime." Basically, he's making the claim that such a crime is largely committed apart from the community's knowledge. Danforth asserts that only the witch and the victim can attest to the guilt of the witch. However, since there is little likelihood the witch will choose to incriminate herself, Danforth maintains that the court must turn to the victims to furnish the truth.
Danforth explains that lawyers cannot be called upon to argue a witch's case; the court already considers the witch guilty, and so there can be no conceivable reason to call upon lawyers to represent a client who has already been judged guilty. This is the strange reasoning Danforth uses when he argues against Hale's request for a lawyer to represent Proctor. To Danforth, Proctor is already guilty, and no amount of arguing on his part will exonerate him (clear him of his guilt). For his part, Proctor makes the argument that Mary Warren has been lying to the court. He presents Danforth a deposition signed by Mary, but Danforth doesn't want to believe that Mary has lied.
Danforth interviews Mary, who confesses that she did indeed lie to the court. Distressed, Danforth proceeds to question the other girls (Susanna Walcott, Mercy Lewis, Betty Parris, and Abigail Williams), but they maliciously contradict Mary's testimony. Meanwhile, Proctor argues that the girls are all "marvelous pretenders" and cannot be trusted to provide an account of the truth. During the proceedings, Abigail supposedly calls forth an evil spirit to afflict the other girls, and the girls scream in terror that they have been seized by cold. Mary Warren tries to run away, and Proctor tries in vain to stop her.
In the end, Proctor seizes Abigail and accuses her of seeking a "whore's vengeance" on him. He begs Danforth and the court to consider the possibility that Abigail is the main instigator behind the lies. So, we can see that Proctor is in a very difficult position here. Because of Danforth's policy against letting lawyers represent witches, Proctor has no recourse to true justice. When Mary Warren later turns against Proctor, he is condemned thoroughly and irrevocably to death. The trials demonstrate that witchcraft is far from being an invisible crime. Yet, officials like Danforth are slow in admitting this because such an admittance would compromise their own positions of influence and power.
He says that the crime of witchcraft is extraordinary. He suggests that accused witches cannot even use lawyers because you either are a witch or you're not; he calls it an invisible crime. The only witnesses can be the witch and the victim. Since the witch will not accuse herself, the court can only rely upon the testimony of the victims, i.e., those who have been affected by the witch.
Danforth is clearly making an absurd argument. His motivation is to prosecute to bolster his standing and get him re-elected. He claims he has seen all kinds of odd behavior in court, so he has no reason to disbelieve what Abigail and the other girls are saying. Danforth ignores the possibility that the girls are lying (pretense) which is ironic because lying about possession is also an invisible crime. So, while Danforth's justification for not representing the accused is just moronic, this is a double standard.