At the heart of his work lies atheism and a belief that human existence is brief, animalistic and often painful. This perception came out of both his objective interpretation of the world and his own experiences, in particular his homosexuality and an attraction to violence. This may have been literally beaten into him in adolescence when his father instructed his Irish groom to horsewhip the boy in an effort to thrash him into normality. No wonder he saw mankind as “nothing but meat”.

The second major strand in his art is aesthetic: what should the painter’s response be not just to the great artists of history but to photography, which had, in many ways, superseded them? Bacon’s answer was to co-opt both paintings and photographs. Lurking in his pictures are images inspired by photographs of Hitler and Himmler getting out of cars, Eadweard Muybridge’s pioneering scene-by-scene stills of the body in motion and the influence of, among others, Goya, El Greco, Titian, Daumier and, of course, Velázquez.

The motif of the screaming mouth – such a potent symbol of pain – most obvious in his interpretations of Velázquez’s portrait of Pope Innocent X, had its origins in Bacon’s own sinus problems and an oral operation he underwent in the 1930s. Its form, however, can be traced to a hand-­coloured book of diseases of the mouth he found in Paris.

Bacon’s status as an artist lies in the complicated and entirely individual way he found to express his complicated psychology. And the Tate show, with pictures from every stage of his career, will confirm his reputation as the pre-­eminent figurative painter of the 20th century – even though his figures teeter on the verge of abstraction. It is revealing of our worried times, perhaps, that his difficult, always uncomfortable and often distressing pictures resonate with so many people.