Rousseau's Dog, by David Edmonds & John Eidinow

When the wisest head in Europe blew his top

Of all stereotypes, the most deceitful is that of the Thinker: taciturn, removed and measured in all things. In truth, great work in philosophy is normally accomplished in a spasm of nervous energy. When the Thinker removes his fist from his brow, he is apt to use it to defend his achievements and reputation as jealously as any other mortal. David Edmonds and John Eidinow first examined the philosopher's capacity for childish behaviour in Wittgenstein's Poker. The book was a hit because although we no longer expect moral perfection from philosophers, we expect them to be at least mature. Whether or not Wittgenstein once threatened Karl Popper with a piece of iron, it would not have been an isolated incident in the history of philosophy. To prove the point, the authors are back with the feud between David Hume and Jean-Jacques Rousseau in the 1760s.

By 1766, Rousseau had been hounded through several countries by officers of the law and stone-throwing mobs, incensed by his revolutionary opinions. Hume was a friend of a friend and, by common consent, the nicest man in Europe. He was the obvious candidate to bear his fellow philosopher to safety in Britain, and escorted him to London against the better judgment of his confidants.

After playing host to Rousseau for four months and setting him up with a house in the country, Hume received a letter from the philosopher accusing him of luring his charge into a trap. Rousseau was suffering from paranoid delusions. He even wrote to the Lord Chancellor asking for a bodyguard to escort him out of England. Hume famously retreated to backgammon with his drinking companions when his philosophising brought him to despair, but was unable to use the same device when his integrity was attacked. This may be a human failing, but not surprising in a philosopher who believed our rewards were in this life, not the next.

Hume had harboured suspicions from the start. Rousseau complained about his health, yet during the crossing he possessed the constitution to stand out all night on the freezing deck. He pleaded poverty, yet was receiving handsome profits from the sales of his books. That Hume gave freely of himself was too galling for his pride when he received a character assassination from one of the world's sharpest thinkers. Edmonds and Eidinow give an insightful account of how Hume's attempt at damage-limitation led him to blight his spotless reputation. In a panic, he lied and tried to turn Rousseau's friends against him. In the end, the public sided with the underdog.

According to the authors, Hume never grasped Rousseau's concept of friendship, which required a bonding between souls. Their judgment is right to a point, but also unfair. A man's talent can deserve homage that his character does not, and Hume would have felt the demands of Rousseau's gifts as profoundly as his flaws repelled him. Rousseau was fooled by his own self-regard into taking Hume's generosity as devotion, and deceived himself again when he took the truth for hatred and jealousy.

"A true friend," the authors write, "had every claim on his heart but none on his liberty". One hopes there are no such friendships, because this is a view in which duty plays no role. No duty that can be fulfilled with as little as a smile is worthy of the name. If that was all Rousseau was looking for, he should have stuck to his dog.

Nicholas Fearn's 'Philosophy' is published by Atlantic