Which is the better way to understand mental illness: a medical textbook, or a novel? Is there any reason to bother raking through three hundred pages of prose, when irregular human behaviour can be reduced to a simple clinical term, such as “bipolar”, or “autism”, or “schizophrenia”?

Nathan Filer thought long and hard about whether to include any of these words in his debut book, The Shock of the Fall, which won the Costa First Novel Award on 6 January. It tells the story of Matthew Holmes, a headstrong, quick-witted teenager haunted by the death of his brother on a family holiday when he was nine years old.

“I don’t diagnose him with schizophrenia, in that I never have a character in a position of authority say he’s got it,” Filer told me shortly after the prize announcement, “but then, he does end up on a community treatment order being given a depot injection against his will, so I guess it’s pretty strongly implied.”

In many ways Filer, a registered mental health nurse who worked for four years on a 19-bed open acute ward for people with a range of mental illnesses, is writing against the trend. The American essayist Marco Roth has charted the rise of the “neuronovel”, in which bad behaviour becomes synonymous with bad brain chemistry: think of Jed Parry in Ian McEwan’s Enduring Love, or Christopher Boone in Mark Haddon’s Curious Incident of the Dog in the Night-Time. For Roth, this represents a loss of confidence in fiction: an allegory of the fear that science has become the best and only reliable measure of the human condition.

There is an increasing sense that inwardness, subjectivity and selfhood – the basic stuff of fiction – ought to be discarded as soon as possible. “I never quite understood why you would read fiction to understand the human condition,” Richard Dawkins told the New Republic last year. As cognitive therapies give way to cheap and readily available pharmaceuticals (the number of people in the UK taking antidepressants doubled between 2000 and 2011), the reduction of mind to brain in literature may well have been foreseen.

“It’s a temptation to write the illness and then bolt the character on to it,” Filer says, “but having worked in the field for so many years, it became a little bit more natural for me to see the person and see the illness as one facet of their character.”

We have been depicting mental illness in art far longer than we have been diagnosing it. Hamlet may have been bipolar, but that is not all that he was. It is not unreasonable to assume that observation and imagination may still paint a broader picture than textbooks, or even help them along.

Filer speaks fluent NHS: he still works occasional shifts as a nurse and uses expressions such as “service user” and “pathologise” without sounding cold or patronising. In his book, Matthew becomes a kind of challenge – to look beyond the disease and see what remains.

“The only way we’re ever able to diagnose people in mental health – we don’t put patients in an MRI scanner – is through the way they interact with people,” Filer says, explaining how he used his novel to ask questions, arising out of his work, on the location of mental illness.

“If we say that his schizophrenia is located in him,” he goes on, “and look at his mother (clearly there’s some anxiety and depression there), it seems that their respective problems are located in the space between them.

“The catalyst for breakdown is always stress: it’s life events. If you want to depict mental illness accurately, then the interplay needs to be shown. Fiction can do that.”