For the third day of the trial, Sinclair calls his only witness to the stand: James Bruce Thomson. Thomson describes his credentials and the so-called “hereditary criminal class” (223), which he believes to be genetically predisposed to criminality. He then moves on to the specifics of the case, beginning with his examination of Roderick at Inverness and his journey to Culduie. However, he shocks Sinclair—and the rest of the courtroom—by testifying that he does not believe that Roderick belongs to a genetic class of criminals.

Sinclair attempts to gain control of the exchange, prompting Thomson to testify that one can be legally insane without exhibiting obvious outward signs of madness. He explains that someone can be morally insane, or in possession of their senses yet nevertheless unable to stop themselves from committing crimes and misdeeds—meaning that Roderick’s own statement doesn’t necessarily preclude him from suffering from this condition. Sinclair asks Thomson whether he believes Roderick suffers from moral insanity, and Thomson replies that he believes Roderick’s actions could indicate such a condition if his version of events could be accepted as true.

Leaving Sinclair shocked once more, Thomson proceeds to propose an alternate explanation of Roderick’s crimes. Thomson argues, based on Roderick’s own inconsistencies in describing the murders, that Flora was the primary target of the attack and that Roderick killed Donnie and Lachlan because they interrupted his attack on Flora.