Final answer:
The likely nature of the patient's illness is a stroke.
Step-by-step explanation:
The likely nature of the patient's illness is a stroke. A stroke occurs when there is a disruption of blood flow to the brain, resulting in brain cell damage and loss of function. In this case, the patient's gradual weakness, paralysis of the right arm and leg, deteriorating vision, and mental faculties point towards a stroke as the cause of his symptoms.