Insanity usually implies mental derangement.

Now lunacy suggests periodic spells of insanity. Just insane once in a while.

Dementia...is an acquired mental disorder...something else gives it to you??? Usually of organic origin.

Amentia... is a congenital mental deficiency. Perhaps mom and dad gave it to you...

Psychosis means all of the above and more...any definitions in which the personality is disorganized.

MOJO3