I have seen people undergo astonishing trauma and emerge scarcely changed by the experience. I’ve seen others thrown off balance by what look like insignificant disruptions: the butterfly’s wing that causes a brainstorm. We cannot expect a system as complex as the human mind to respond in predictable or linear ways. We tend to deploy vast psychological resources to prevent ourselves from changing: sometimes wisely, when despair or madness loom; sometimes foolishly. For fear of the tempest, we can deprive ourselves of experience, and deprive others of the changes we need to make to become better people.

Knowing all this, I’m still astonished by how small a thing it was that shaped my life. In August, I wrote an article for the Guardian’s “A book that changed me” series. My contribution was peculiar in that I didn’t have a title. I had found the book when I was eight, in a dusty box in my boarding school’s sanatorium, and returned to it whenever I fell ill. It was the first countervailing influence in a childhood surrounded by conservative thought and Conservative politics, and it had a profound effect on me. I described the book briefly and – it turns out – not very accurately. And I asked the readers for help.

The result was the internet at its best, and its worst. Many readers came forward with suggestions, and others generously went to some lengths to find it. Some people recommended fascinating books that had changed their own lives.

The same comment thread was flooded by a great tide of vitriol along these lines: how dare you have leftwing views/tell us about your life/presume to exist after attending a private school?

Well, the thing about childhood is that you don’t control it: it is simply what surrounds you. To attack someone for their origins is no more rational than attacking someone for their gender, height or colour of skin.

We could wait until we have been reincarnated into the appropriate socioeconomic class, or we can make the most of the life we have, using it as far as possible to do good and confront harm. The left would be a bleaker place without thinkers from privileged backgrounds. You could disown the likes of George Orwell, Tony Benn, Peter Kropotkin, Friedrich Engels, Elizabeth Fry, Leo Tolstoy, William Morris, Beatrice Webb, Mahatma Gandhi, Alexandra Kollontai, Bertrand Russell, Vera Brittain, Clement Attlee, William Beveridge, Franklin Roosevelt, Paul Foot and Millicent Fawcett because they came from the wrong place. Or you could judge a person by what they do rather than by how they were born.

But to return to the main point, I owe a big apology to some of the readers. Quite early in the thread, several people, I now discover, correctly identified the book. But when I looked at the cover image and other illustrations I found online, they triggered no recognition.

So people continued searching. Only when, after the comments closed, someone sent me a photo of a particular page did my memory suddenly flare into life.

It was a book called Paolo and Panetto by the exiled Austrian writer Bettina Ehrlich, now long out of print. I ordered an old copy. As soon as I handled it and smelt it, a wave of confused feeling crashed over me. All at once, I felt the joy of its discovery, the intrigue and absorption with which I first read it, and the misery of lying in a cheerless sick bay far from anyone I loved. As if to exorcise that ghost, rather than read it alone I curled up with my three-year-old and read it to her. She loved it as much as I had done.

Two things struck me. The first was the patchiness of my memory. While I had recalled the bare bones of the story, I had collapsed two of the leading characters into one, excising the most interesting person – the young god Pan – from the narrative. In my memory, a spoilt child from a plush apartment met another boy on the streets picking up fag ends at night. This boy led him into the countryside and introduced him to unfamiliar freedoms. But in the original, it was a girl whom the rich boy met on the street. Through her he met Pan (or Panetto), who took him into the fields and woods. This reminded me of how untrustworthy our convictions are. We use our imagination to fill gaps of which we are unaware. We are unreliable witnesses to ourselves.

The second realisation was that the story was milder than I had remembered. In my mind, it had become a radical commentary on inequality, perhaps written by an Italian communist. But the book wears its politics lightly. At the beginning, the spoilt child and the ragged girl explore the stark contrasts between their lives. This theme is then dropped until the end, when Paolo throws his father’s cigarettes out of the window for the street children to collect.

That’s all there is: a privileged but neglected boy steps out of his insulated world, encounters a child living in poverty and seeks to redistribute a tiny fraction of his family’s wealth. It was this tremor of a butterfly’s wing that began a process that was to change my life.

It was not, of course, the only event. Hundreds of later influences helped to shape my mind, and continue to do so. I hope the process never stops. For me, living a full life means being open to experience and persuasion, experimenting endlessly with new arguments and knowledge, risking ridicule by testing new ideas. The thought of descending into static respectability appals me.

Dante was right. The inner circle of hell is a place where nothing changes, where life is frozen into immobility, where no one can change their destiny. When people claim that you can’t teach an old dog new tricks, or that a leopard can’t change its spots, they shut themselves out of life.

To inherit social position, wealth and opportunities: this is bad enough in a society with democratic pretensions. To inherit ideas and framings as well is to renounce not only intellectual honesty, but also much of our humanity.