A century ago, even the greatest mind doctor of them all, Sigmund Freud, predicted that one day mental illnesses would be explained physically, once scientific techniques for the study of the brain became possible. In a typically prescient but off-the-mark way, he even experimented with treating patients with a chemical remedy – cocaine.

All these years later, Freud’s conjecture is bolstered by firm evidence. A psychiatrist, Tim Cantopher, says that if he were to draw fluid from the spinal cord of depressed patients he would find a deficiency of two chemicals: serotonin and noradrenaline, essential neurotransmitters that regulate a host of functions in the body and brain.

“Depressive illness is not a psychological or an emotional state and is not a mental illness. It is a physical illness,” Cantopher has written. “This is not a metaphor; it is a fact.”

If such a categorical statement can be made about depression – and it may be even more applicable to other psychological problems – it raises the question of why the term “mental” illness is used at all. Why are depression, schizophrenia, psychosis, alcoholism and personality disorders dealt with by mental health services, separate from the rest of the NHS?

If this sounds like organisational nit-picking, it is not. There is plenty of evidence that lack of integration of mental and other health services serves patients on both sides badly. Meanwhile, institutionalising the idea that “mental” health is somehow different from other illnesses perpetuates the idea that it these problems are “in the mind”, that perhaps patients could buck up a bit, or they are untreatable, to be feared, even avoided. Not only is such stigma hurtful and unhelpful, often discouraging sufferers from seeking treatment, it also has created an environment in which, as a nation, the UK accepts appalling discrimination.

Last month, politicians announced waiting targets for mental health services – a decade after they were introduced in the rest of the NHS. Up until now, patients would see a GP and be told to wait for mental health services to get in touch. They might be prescribed antidepressants immediately, but all the evidence suggests a combination of drug and talking therapy – a key part of which can be learning to manage one’s condition – is the most effective treatment. After diagnosis, however, therapy might only be available after many months and waiting more than a year is not uncommon. Earlier this year, the incoming president of the Royal College of Psychiatrists, professor Simon Wessely, estimated that, at any given time, two-thirds of Britons with depression were not undergoing treatment. Some children with the illness had had no treatment at all – “literally none”, he claimed. And while patients wait, one in six of them will try to kill themselves, according to a new report by the charity Mind.

Wessely wonders what it would be like if 70% of people with cancer were not getting treated. Or if one in six patients with HIV tried to commit suicide while they waited for treatment. There would be a national outcry.

After centuries of accusing people displaying signs of mental distress of possession by demons or witchcraft, western civilisation underwent a fundamental change in its approach to these problems just over a hundred years ago, when the German Emil Kraepelin suggested psychological problems had a physical basis – though he didn’t know what it might be. Freud went further, arguing that psychological and physical functions could not be separated. The use of chemicals to help treat such conditions emerged after the second world war. Most recently, scientists claim to have identified gene variants linked to mental illnesses.

Arguably, some psychiatrists have been driven to an obsession with biology – perhaps by the spending power of the drug companies. The risk of this is that it draws professional attention and research away from wider psychological, social and environmental causes – and remedies. Despite decades of an increasingly biological approach to mental health, claims University of Liverpool professor Richard Bentall, the prospects for patients are no better than they were in Victorian times.

But to feel that doctors have gone too far in blaming biology is not to say it does not play a role: these arguments are largely about different weights of attribution, as experts try to understand the complicated interplay between a patient’s physical makeup and their life experiences. Carmine Pariante, professor of biological psychiatry at King’s College London’s Institute of Psychiatry, describes his work as understanding where people lie at different points on the “resilience spectrum”, with some experiencing mental illness in response to stressors, and others not.

Nor are other health problems so different: doctors, scientists and public health experts are increasingly aware of environmental, social and psychological issues behind other illnesses, most publicly heart disease, diabetes and some cancers. To take a purely biological approach to diabetes would also be a failure, but that does not make it anything other than a physical illness.

A meta-study of published international research found no good evidence that explaining that mental illnesses are biological reduces social stigma. In the case of schizophrenia the opposite can happen – though more nuanced, mixed explanations might work better. However, the same research found signs that biological explanations could reduce “self-stigma”, including blame and guilt, and in several studies patients who were given these accounts of their illness were more likely to seek proper medical help. There were also studies in which a wider understanding of the physical nature of mental illnesses appeared to reduce “structural discrimination”, including poor relative funding.

In the last few years attempts have been made to improve mental health services. Two years before the new waiting list targets were announced the UK government decreed mental health patients should have “parity of esteem” in getting treatment.

The most recent NHS England five-year plan admitted a key to future success was to “dissolve the classic divide” between mental and physical health, and envisaged more “liaison psychiatry” where psychiatrists work across hospital wards, which in trials has notably reduced readmissions.

But it is hard to believe real progress can be made when there are still deep institutional divisions. Not one of 17 NHS board members has a medical background in mental health and only one – a non-executive director, Lord Adebowale – lists professional experience in the field.

In Birmingham, the Conservative MP and chair of the all-party parliamentary group on mental health, James Morris, has personally insisted on mental health advocates sitting on the boards of local clinical commissioning groups, but there is no formal requirement for this. Last year local authorities in England spent just 1.4% of their public health budgets on preventing mental health problems, perhaps influenced by Department of Health literature, which makes a solitary reference to the issues in 39 pages of guidance.

Reuniting health services would not be without its own issues, practical and economic. But that the current regime is failing is obvious. And as research continues to build on Freud’s instinct, the case only gets clearer. Parity of esteem sounds like a gracious concession; in fact, it’s a scientific necessity.