Judgement should be suspended initially in clinical assessment of the rationality or otherwise of even bizarre-seeming behaviour and experience. Doing so in a thoroughgoing way may make the most mad-seeming actions and experience intelligible. Such reactions may even come to be seen as a reasonable response to an unreasonable situation.
As Aaron Esterson said (see previous post), some people who are labelled mad are mad by any criterion. Yet, some are misdiagnosed, and may be confused into believing they are mad or mentally ill, even thinking they have a brain problem, when they have not. Psychiatric diagnosis is not an exact science. And, other people can be so emotionally distraught they seem mad. Even mad people are not necessarily mad in the way they are often said to be by mainstream psychiatry (see eg. my presentation).
Mad people sacrifice shared understanding with others for the internal logic of their own psychotic thought and experience (see previous post). Delusions are not necessarily un-understandable, nor always "bad for us", nor even particularly an exceptional way of thinking (see another previous post). We actually believe all sorts of things, partly often because there isn't clear evidence one way or another or it’s not clear what the best view should be for some of the things we believe. Power relations in society can also determine what we believe.
As Foucault said, reason questioned the rational foundations of what is accepted as reason in the Enlightenment and was the foundation for the origins of modern psychiatry (see eg. previous post). We need to remind ourselves that the distinction between reason and unreason is not absolute. Justin Garson argues that defining madness as not merely the absence of reason is a more positive way of viewing madness. He says it helps to sustain Mad Pride, mad resistance and mad activism. Such a position foregrounds user/survivor perspectives and can pose an inherent challenge to the biomedical dominance of psychiatry (see eg. my review of Mohammed Rashed’s book). Certainly reducing functional mental illness to brain disease is not helpful.