A new film, The Electrical Life of Louis Wain, starring Benedict Cumberbatch, comes to cinemas this week. Its arrival has been accompanied by the suggestion that the illustrator it portrays, who suffered from mental illness, was a dishevelled “mad artist” – and thus a classic example of “outsider art”.

Wain (1860–1939) became well known for his whimsical cat illustrations, which depicted caricatures of the furry pets in human clothing and activities. In fact, like (say) Vincent van Gogh, he was not in the strictest sense an “outsider artist” at all, rather a trained professional who became mentally ill. Outsider artists can come from any background except, by definition, Wain’s: they are the “ordinary geniuses” of everyday life who transcend their backgrounds to create visual work that is uninfluenced by training or association with the mainstream art world. Their visionary work reflects their emotions, and is considered by its appreciators to be the purest form of visual expression yet discovered.

It is true that our awareness of outsider art has its origins in the late 19th and early 20th centuries, and what were then known as “lunatic asylums”. Of course, outsider art – an art produced compulsively by self-taught creators – must have existed for hundreds of years or more, even though no examples were valued or preserved. But a few enlightened hospital psychiatrists were the first to see that certain patients produced work of great originality and power. Those psychiatrists kept the drawings that had been routinely swept away, and the first collections were gradually put together.

These items were not necessarily considered “art” as much as diagnostic aids or specimens that illustrated various conditions. Some early examinations of patients’ work took place in the 19th century, but it wasn’t until 1922 that the first serious study of psychiatric expression, Bildnerei der Geistesranken, was published by Hans Prinzhorn. His book, with its mass of illustrations and theories of creativity, was the first to herald patients’ works as art, and the 10 “schizophrenic masters” celebrated in his pages were judged to have created works as valid as any other mainstream forms.

Not everyone agreed with this assessment. In Germany, it was opposed by Adolf Hitler, who saw in Prinzhorn’s collection the proof that modern art was tainted by madness and degeneracy. The works from Prinzhorn’s collection were duly exhibited alongside those of Franz Marc, Oskar Kokoschka and Paul Klee to prove the connection, in the series of Entartete Kunst (“degenerate art”) exhibitions that took place across Germany in the late 1930s.