A more thorough explanation:

Definition: Monomania is a type of insanity where a person becomes obsessed with a particular subject or group of subjects. This obsession is usually shown through a single delusion that the person believes to be true.

For example, a person suffering from monomania may believe that they are being followed by aliens or that they have a special power that no one else has. They may become so obsessed with this delusion that it affects their daily life and relationships.

If someone suffering from monomania creates a will, it is usually considered valid unless it can be proven that the delusion influenced specific provisions in the will.

Example: John was diagnosed with monomania after he became convinced that he was a famous musician. He would spend hours each day playing music and talking about his supposed fame, even though he had never played a concert or released any music. His obsession with this delusion caused him to lose his job and his relationships with friends and family.

Explanation: This example illustrates how monomania can cause a person to become obsessed with a delusion that is not based in reality. John's belief that he was a famous musician was a single delusion that he believed to be true, even though there was no evidence to support it. This obsession caused him to lose his job and relationships, which is a common consequence of monomania.