Iron rusts from disuse; stagnant water loses its purity and in cold weather becomes frozen; even so does inaction sap the vigor of the mind.

More from Leonardo da Vinci

• For once you have tasted flight you will walk the earth with your eyes turned skywards, for there you have been and there you will long to return.
• I have learned from an early age to abjure the use of meat, and the time will come when men such as I will look upon the murder of animals as they now look upon the murder of men.
• Lying on a feather mattress or quilt will not bring you renown.