From
王安石's
《题张司业诗》 (Song dynasty):
看似寻常最奇崛,成如容易却艰辛 (What looks commonplace is most extraordinary; what seems effortless was hardest won). Wang Anshi's praise of Tang poet
张籍 - plain-style poetry that required enormous craft. The couplet is a favorite motto in Chinese aesthetics for the ideal of
平淡 (understated brilliance).