faculty of reasoning

In medicine, you come to recognize how unreliable the faculty of reasoning is and how susceptible it is to subtle forms of exploitation.

For they do not appear to me to have lost the faculty of reasoning, but having joined together some ideas very wrongly, they mistake them for truths; and they err as men do that argue right from wrong principles.

But this inability to exert self-control, which the sheriff has described as an inability to complete the reasoning process, must be distinguished from the essential requirement that there should be total alienation of the accused's mental faculties of reasoning and of understanding what he is doing.