I just read Flourishing, the first part of Isaiah Berlin’s letters. The first impression that springs to mind is the banter, the spirit, the cascade of words.
An example of his unique style can be found in a letter to Elizabeth Bowen, 2 January 
Thank you very much indeed for your letter: it arrived at a time when I was mildly depressed by two or three letters from old friends who patently accused me in the kindliest possible manner of totally lacking in sensibility or capacity to understand either them or their past present and possible relations with other old friends. This, which regularly happens, automatically produces in me first fury then petulance, then pathos, then an artificial and pompous resolve to accept my own inhumanity as an unignorable but somehow also rather grand fact, & apply myself exclusively to my own proper abstractions, with special attention to Descartes’ arguments to prove that animals are automata, with a view of extending this hypothesis to my correspondents who have been so vulgar as to accuse me of what I can coldly point to as a virtue which they can never know.
Even as a relative young man (28) he shows wisdom, sensitivity and his unique style
again in a letter to Elizabeth Bowen July 1937.
You are the only person […] towards whom, apart from other, simpler & warmer feelings, I feel absolute trust and respect and admiration […]. Remarks about absolute trust usually lead up to something, are bullying in a peculiarly awful way, a prelude to burdening with something particularly disagreeable to bear. In this case they prelude nothing. And indeed I already feel a revulsion from the sentimentality of the words, which please abstract: the meaning is not obscure, and I cannot see why it should not be stated.
The title of this first collection of letters is derived from a cable Isaiah sent to his parents: [I am] flourishing. This was a lie, because when he sent the cable he was lying in hospital. Isaiah went to some lengths to convince his parents that his health was ok. They didn’t believe him. The funny part is that the censor — this was in the war — withheld the cable for some days because he thought there was a hidden message.
In the pre-war letters Isaiah is witty, funny and gossipy. The letters become much more interesting when he is working for the Foreign Office in Washington. The amount of people he met is tantalizing. His confidence is growing and he is making a circle of life-long friends.
I was particularly interested in his period in Moscow and Leningrad where he met Pasternak and Anna Achmatova. Especially the two meetings with Achmatova had made a lasting impression on both of them, but he hardly mentions the encounters in his letters. Without doubt he omitted to write about her to protect her.
I am looking forward to read the second volume of letters: Enlightening.
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