鏡花水月
[詞典] flowers in a mirror and the moon's reflection in water -- a mirage, insubstantial objects; reflection of a flower in the mirror or of the moon in the water; moonlight in the water; beautiful but evanescent dreams;
[例句]然而，煩躁的都市啊，請不要忘記，這世界本是鏡花水月。
However, whiny urban, please don't forget, the world is an illusion.