Casey Kasem -- His Mind Is Gone

Casey Kasem is in terrible shape ... his mind is almost gone and he's unable to function -- this according to new legal docs obtained by TMZ.

The document outlines the mental state of the legendary founder of "America's Top 40" radio program. According to the examining physician, Casey has

-- major impairment to short term, long term and immediate recall memory

-- major impairment to verbally communicate

-- major impairment to perform simple calculations, plan simple tasks and reason logically.

-- severe disorganized thinking

-- moderate hallucinations

The document was filed by Julie, one of Casey's daughters who is trying to become conservator. Casey's wife of 33 years, Jean, is fighting the move to take control away from her.

The doctor says Casey has been sliding downhill for the last 3 to 4 years. He's suffering from advanced Parkinson's and dementia.