It is quite possible that personal diaries existed in pre-Ming China, but none have survived over the centuries precisely because they were so personal. Most ancient Chinese books that have lasted to today were preserved because the imperial court considered them worth preserving, and therefore collected copies of them in the imperial library. On the other hand, a personal diary would neither have been accessible to the imperial court nor considered interesting enough, since it was the public career of a man that court historians were interested in recording. If they wanted some insight into his private thoughts, they could always look in his poetry or his letters (which was a part of one's personality appropriately 'packaged' for public consumption). Apparently no Chinese historical personage expected or wanted his diary to become available to future generations.
This seems to have changed in the Ming and Qing dynasties - we have the diaries of Xu Xiake and Zeng Guofan, among others. Can anyone suggest reasons for the new interest in diaries?
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Chinese history recordings began in around 800 BC, and historians usually kept a 'daily record' of what happens in court. Can we say that is a form of diary?
This was the Qiju Zhu 起居注 (Notes on Daily Events), kept by the emperor's personal secretary. It recorded every detail of the emperor's day, including the state of his bowel movements, for the benefit of future court historians. Of course, it wouldn't contain much in the way of the emperor's private thoughts either.