Iron rusts from disuse; stagnant water loses its purity and in cold weather becomes frozen; even so does inaction sap the vigour of the mind.
铁器若不常用就会生锈，水若不流动会变得污浊，冷天则会结冰；懒于思考的话，思想也会失去活力。

——Leonardo da Vinci
——莱昂纳多·达芬奇