Entire books have been written about psychiatric diagnosis. Diagnosis comes from a Latin application of the Greek: ‘gnosis’, to know and from the Greek: ‘dia’, through or thorough. To diagnose something is to have thorough knowledge of it.
Dx of physical ailments
The means of understanding of a physical ailment is quite different from the way one comes to understand a mental ailment. For examples, if one falls and hurts one’s arm, a practitioner will probably order an X-ray examination. A radiologist will be able to see if a bone has been fractured. Further treatment will be based on what the radiologist sees. Similarly, fluid on the knee, fluid in the lungs, enlarged hearts and even bacterial infections can be detected with various tests. There are no such tests for psychological or psychiatric ailments such as depression, anxiety and mania.
Dx of mental ailments
There is a school of thought that questions the existence of mental illness. This was noted in a recent article in The Guardian. Forget, for a moment that the current battle was triggered by DSM V, the latest version of the Diagnostic and Statistical Manual of Mental Disorders, released May 2013; similar views existed in the late 1970s according to Dr. Hagop Akiskal, in the Introduction to Psychiatric Diagnosis. To understand the argument better, I define the term “reductionism”.
According to Wikipedia, “Reductionism is a philosophical position which holds that a complex system is nothing but the sum of its parts…” Merriam Webster’s version addresses our situation specifically: reductionism is the “explanation of complex life-science processes and phenomena in terms of the laws of physics and chemistry.”
Dr. Akiskal, describes the reductionist approach to psychiatric phenomena. He writes, “According to this viewpoint, psychiatric phenomena can either be reduced to neurologic dysfunctions or else conceived as psychosocial problems of living. Such a simplistic model fails to take into account the fact that man is a psychobiological unit and that psychiatric disorders, by their very nature, typically involve dysfunctions within this psychobiological interface.”
I find this concept very compelling: an interface of the physical and psychic. I use the term ‘psychic’ to mean ‘of the mind’.
So how does one begin to make a diagnosis of something that cannot be objectively tested for? A system which includes biologic and behavioral phenomena should be developed; correlating biologic phenomena with psychiatric syndromes would produce clinically meaningful predictors.
Akiskal relates a story which illustrates the need for consistent diagnostic criteria. After publication of comparisons between psychiatric diagnoses in the US and the UK, a writer noted that “American schizophrenics could be ‘cured’ by crossing the Atlantic.” 
One can see the importance of establishing consistent diagnostic criteria; accuracy is also required, so that appropriate treatment may be rendered.
 Akiskal, H.S., Webb, W.L. eds. Psychiatric Diagnosis: Exploration of Biological Predictors SP Medical & Scientific Books, New York 1978 p2
 Akiskal, H.S., Webb, W.L., eds. Psychiatric Diagnosis: Exploration of Biological Predictors SP Medical & Scientific Books, New York 1978 p3
 Ibid p3
 Ibid p3