dementia

Dementia (meaning "deprived of mind") is a serious loss of cognitive ability in a previously unimpaired person, beyond what might be expected from normal aging. It may be static, the result of a unique global brain injury, or progressive, resulting in long-term decline due to damage or disease in the body. Although dementia is far more common in the geriatric population, it may occur in any stage of adulthood.

I understand it as someone losing logic.