A more thorough explanation:

A declaration of state of mind is a statement made by an individual about their current mental or emotional state. It can be used as evidence in legal proceedings to show the person's state of mind at a particular time.

For example, if someone is accused of a crime and claims that they were not in the right state of mind to understand their actions, a declaration of state of mind could be used to support their defense. Similarly, if someone is making a will, they may include a declaration of their state of mind to show that they were of sound mind and understood the implications of their decisions.

Overall, a declaration of state of mind is a way for individuals to communicate their mental or emotional state in a formal and legal context.