What is the origin of the idiom "all there" and when to use it?

The idiom "all there" has its origins in the idea of mental capacity or intelligence. The word "there" in this context implies the mental state or condition of a person. It likely evolved from earlier idiomatic expressions that used "there" to refer to a person's mental faculties, such as being "all there" or "not all there." This phrase is typically used to describe someone's mental or cognitive state. When someone is said to be "all there," it means that they are mentally sound, alert, intelligent, and in full possession of their mental faculties.