"Bereft of reason" is an expression used to describe a person who has lost their ability to think rationally. Some synonyms for this phrase include "deranged," "insane," "mad," "crazy," "delirious," and "unhinged." These words can be used interchangeably to convey the idea of someone who is not in their right mind. Other related words include "delusional," "psychotic," "maniacal," and "lunatic." These words are often used in a negative or derogatory manner, and it is important to remember to use language that is respectful and humane, particularly when describing mental health conditions.