What does it mean to say psychotic experiences are symptoms of an illness?

I have been reading the report by the Division of Clinical Psychology Understanding psychosis and schizophrenia. The report states that calling experiences, such as hearing voices and feeling paranoid, symptoms of mental illness is "only one way of thinking about them, with advantages and disadvantages" (p. 6). It says that "not everyone agrees that there is an underlying illness" (p. 7). It goes on, "The idea that they [experiences such as hearing voices] are symptoms of illness, perhaps caused by some sort of chemical imbalance or other problem, is just one of the theories. There is no objective biological test such as a blood test or scan for diagnosing mental illness" (p. 17).

In the section on the advantages of seeing such experiences as mental illness, Laura Lee, one of the contributors to the report, is quoted as saying, "If the concept of illness was extended from biology to include our emotional/spiritual/thinking and meaning-making faculties we would have a holistic approach which would offer more" (p. 25). I agree with Laura (although may not make as much of the spiritual reference). I think the rest of the report could have been clearer on this point. There's nothing intrinsically wrong with using the term 'mental illness'. Although the terms illness and disease are used interchangeably, there are specific meanings. To quote from Eric Cassell in The Healer's Art, "Disease is something an organ has; illness is something a ... [person] has". The error is to think mental illness is a brain disease.