From Zhu Xi's (
朱熹) Song-dynasty philosophical poem
观书有感·其一:
半亩方塘一鉴开,
天光云影共徘徊。
问渠那得清如许?
为有源头活水来 - 'A half-mu square pond opens like a mirror; sky and cloud shadows drift together. How does the pond stay so clear? - Because fresh water keeps flowing from the source.' A metaphor for how the mind stays clear only through constant study.