In this book called "Yua Ku Khong Ku" which can be literally translated as 'I' and 'mine', Buddhadasa pointed out the need to abandon this sense of begin 'I' and 'mine'. It is the original source of selfishness. He also talked about 'sun-na-ta' - 'voidness' or 'emptiness'. The realzation of 'sun-na-ta' is the highest level of Buddhist practice because is 'sun-na-ta', no 'I' and no 'mine' exist. It is Nibbana.