Peppiatt notes the shift in the subject matter in Bacon's work. 'When I was young I needed extreme subject matter for my paintings', he said, 'Then as I grew older I began to find my subject matter in my own life'. Thus a movement away from screaming Popes and anonymous figures to portraits of his friends. History and mythology is supplanted by portraiture.

They were his close friends, whom he saw on an almost daily basis: he had watched them in and out of love, drunk and sober, close up and across the street, in snapshots and in mirrors. He knew their faces better than they did themselves; he recalled and rehearsed the shadows case as they laughed under bar light, the dark pools on the reddened flesh, then the head snapping back in a blur, as if in a search of its old contours.

He saw them in film sequence, always moving, and at times almost unrecognisable from one instant to the next. This was the mystery of their appearance, which he loved and which prompted him to try and capture them, tenderly, violently, erotically, in paint.

[….] The artist appears to have identified with Isabel Rawthorne, for instance, whom he painted with obsessive frequency, both in full-length portraits and small-format heads. If a magnificent sense of dignity emanates from these studies, it is because the artist's affection is greater, but only just, than the destructive fury with which he dislocated and twisted her every feature.

Bacon once said that he thought of real friendship as a state in which two people pulled each other to pieces – dissecting and criticising mercilessly. This is the act of 'friendship' that Bacon perpetrates in the portraits: a pulling apart of the other until he gets to an irreducible truth or 'fact' as he liked to call it, in a pseudo-scientific fashion – about their appearance and their character.

Peppiatt, Francis Bacon: Anatomy of an Enigma