“Those who are in love with practice without knowledge are like the sailor who gets into a ship without rudder or compass and who never can be certain whether he is going. Practice must always be founded on sound theory.”
Quote attributed to The Notebooks of Leonardo Da Vinci, Volume I, at Constable.net and at likewise attributed at PublicLiterature.org. You can download and read the complete version of The Notebooks of Leonardo Da Vinci at Project Gutenburg.