I saw it daubed on a wall the other day: "I Wasn't Here." How's that for existential angst? Philosophers could spend days discussing it. The author is unknown, unlike a notable etching at the ancient Temple of Poseidon at Cape Sounion, Greece. Lord Byron was so impressed by the place he scratched his name on a column. So have many other people since. But vandalism is not vandalism when committed by someone famous. Exhibit A: Byron. Exhibit B: Banksy. Exhibit C: John Stuart Mill.

Mill, the 19th century intellectual and author of On Liberty? Yes, him. He was also a scribbler. A man who left copious annotations on the pages of books written by other people. I was taught that this was a crime; that books should be treated with reverence and respect. Mill probably believed his own works deserved no less. But other authors' books were different. "Stupid. Very stupid," he wrote on a page of Ralph Waldo Emerson. "Nonsense", was his comment on another intellectual's work.

Researchers at the Oxford library that houses Mill's extensive book collection have been busy recording what they call Mill's "marginalia". Instead of denouncing his jottings (in ink, so they're still legible 145 years after his death), academics have called them a goldmine for scholars. As one put it: "he felt strongly, he expressed himself ungenerously."

He also didn't care about anyone who wanted to read his books after he'd finished with them.