A 37- year- old never- married male teacher presented with the complaint that he was“tired of livmg”and was considering“ending it all”. He said he probably was‘‘bom depressed’’because he had never known any happiness and joy and much of his life had been“wasted”and all human existence for him was a“tragic mistake of God”. He was known to be a dedicated and talented teacher, but he felt all his efforts had been“useless and in vain”. He stated that he had always functioned at a“mediocre level”（which was at variance with the good feedback students had given him year after year）;hut did admit he“appreciated work,because there was nothing else to do”. We could not document any major affective episodes. He denied alcohol and drug habits. The most likely diagnosis is（ ）