In our series marking the 500th anniversary of the Reformation, the writer says it is time to integrate mental and physical health

The problem we have with talking about mental health is that we still don’t think of it as an equal priority with physical health. This is wrong not simply because it leads to less money being spent on mental health service provision by governments, but also because it fails to see that the whole idea of mental health shouldn’t be an isolated one.

As a species, we love to divide things up. We draw a straight line in a map between the Atlantic and Indian Oceans while the water remains oblivious. We also draw a line between the mental and physical and base our entire system of healthcare on that false division.

Once upon a time, the medical world detailed the makings of the human body by saying there were four distinct humours. Every single health complaint could be explained as an excess or deficiency of one of four distinct bodily fluids – black bile, yellow bile, phlegm and blood. And these in turn were related to the four elements, as well as the four seasons and the four ages of man.

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If in those golden olden days you were feeling depressed, or melancholic, that was down to an overload of black bile. In fact, the very word melancholia, as with melancholy, stems via Latin from the ancient Greek words melas kholē, which literally meant “black bile”. It seems funny, now, this idea. But in one way at least it was more advanced than much of our present approach. Namely, it did not see a rigid divide between physical and mental health. Mental illness was seen to have physical roots, and was associated with the four elements.

Now, of course, if you visit your doctor about persisting feelings of worthlessness and despair and the futility of existence they will be unlikely to talk of bile of any kind. But then, before we think we are at the end of progress, and have all the answers, we should remember that our doctors might not talk about your body at all.

And yet everything I have come to learn about my own experience of mental health and illness has taught me that this is also a mistake. Even if you have the idea that mental illness is solely an illness of the brain, the brain is a physical thing. The brain is the body. Mental health is physical health. Bodies and minds interact.

You can’t draw a line between a body and a mind any more than you can draw a line between oceans

I would go even further. I don’t want to get too cross-legged on a Himalayan mountain top, but mental health isn’t just brain health. Mental health is intricately related to the whole body. And the whole body is intricately related to mental health. You can’t draw a line between a body and a mind any more than you can draw a line between oceans.

It’s comforting to realise that many cognitive scientists these days acknowledge this. Thoughts aren’t just the products of brains, and vice versa. As Guy Claxton – himself a cognitive scientist – writes in Intelligence in the Flesh, “the body, the gut, the senses, the immune system, the lymphatic system, are so instantaneously and so complicatedly interacting with the brain that you can’t draw a line across the neck and say ‘above the line it’s smart and below the line it’s menial’.” In short: “we don’t just have bodies. We are bodies.”

The word “holistic” is so often associated with scientifically dubious kinds of therapies, but the science is slowly leading us towards a more holistic view of minds and bodies, so our healthcare needs to acknowledge that. We need to realise the physical nature of mental illness and the mental nature of physical illness. Mental hospitals and physical hospitals should all be mental-physical hospitals (but maybe they should be called something catchier).

A happy side product of erasing the line between mind and body, a line that has been boldly drawn since Descartes, would be to destigmatise mental health by placing it on an equal footing with stigma-free physical issues such as asthma and arthritis. It would also lead to a better health service. If mental health was understood in physical terms, it would stop being the poor relation of health when it comes to government funding.

Ultimately, it wouldn’t just help doctors and nurses to understand us better. It would also change the way we view ourselves. The idea that our minds are in our control, and that free will is all, still pervades, and makes people feel a kind of guilt or shame for being ill. A guilt that in itself exacerbates symptoms. We need to truly understand the way minds and bodies interact with each other, and how both are affected by the world.

So a new, more integrated, healthcare system would not only be good because it would help patients, it would also help anyone feeling distress to understand that there is no more shame to be felt than if they had tonsillitis. Illness is illness, and health is health. There can be no “mind over matter” when we understand that mind is matter.

• How to Stop Time by Matt Haig is published by Canongate. To order a copy for £7.99 (RRP £12.99) go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.