The power and passion of this nude fired me at a critical moment when I was getting to grips with a central and tricky theme of the book. But why? I won't keep you guessing. Degas's study of a young woman seems absolutely direct: an observation inflamed by desire. But it is also a homage to Michelangelo. Her pose is closely modelled on male nudes that Michelangelo did in competition with Leonardo da Vinci in Florence in 1504-6.
The Degas drawing translates Michelangelo's male bodies into a female image. And, to be blunt, that helped unlock my own appreciation of the erotic power of the youthful art of Michelangelo. I refuse to see the "nude" as being different in some elevated way from the "naked". Eroticism and intimacy are inherent in any strong depiction of the human body, but writing about the nude is tricky. You can't really do it unless you acknowledge your own feelings – which, I suppose, is the reason the pleasant fiction of the sexless nude was invented by 18th-century critics: to avoid embarrassment.
The drawing by Degas helped me recognise the sexual nature of Michelangelo's art. So the National Gallery helped me with the book, just as it has helped me to learn about great art, and it will mean a lot to be speaking here about The Lost Battles on 15 November. I promise not to be too embarrassing, either about nudes or about my love of the gallery.
• Jonathan Jones will be the speaker at the National Gallery's lunchtime talk on Monday 15 November from 1-1.45pm.