The defence of insanity is covered in the M'Naghten rules. The main rule is that everyone is considered sane until contrary is proved.
There are three elements that need to be satisfied in order for someone to use this defence.
The first element is establishing whether the defendant was suffering from a disease of mind. Disease of the mind is in legal terms and not medical term. It can be a physical illness or mental illness that affects the mind.
In R v Kemp, the court of appeal held that the law was not concerned with the disease of the brain, but the disease of the mind. In case of Sullivan, the house of lords decided that a disease can be any part of the body as long as it affects the mind. So, the disease can be 'functional';affecting the functioning of the defendant, such as in schizophrenia and manic depression. However, the disease could also be organic; that is the disease of the…