It's hard for us to appreciate just how dangerous having an idea could once be – especially if that idea were voiced in the company of others. Philipp Blom's account of the friendship between two radicals of the Enlightenment era, Denis Diderot and Baron D'Holbach, reminds us of the courage of individuals facing imprisonment, torture or even death for the simple expression of a thought.
Even Enlightenment radicals did not have endless pools of courage to draw on, however. One reason, Blom suggests, that Diderot never produced a "substantial philosophical work" was his terrifying imprisonment upon publishing "Letter on the Blind", an attack on the notion of an invisible Creator. He was made to promise never to publish anything blasphemous again, on pain of imprisonment without hope of release. It didn't stop him working on his encyclopedia, but that was banned on publication – an act that "came close to crushing the voice of the radical Enlightenment".
Holbach, who ran dangerous salons in Paris attracting free-thinkers such as Jean-Jacques Rousseau and David Hume, was also a close friend of the radical John Wilkes, who supported US independence. Blom writes about the frisson these meetings caused, and likes to speculate that Diderot, Holbach and Benjamin Franklin met in the salons of the rue Royale – but, with no paper trail, we can only imagine what conversation between them gave birth to a new nation.