Samuel Johnson, from his “Life of Milton”:

It is told that in the art of education [Milton] performed wonders, and a formidable list is given of the authors, Greek and Latin, that were read in Aldersgate-street by youth between ten and fifteen or sixteen years of age. Those who tell or receive these stories should consider that nobody can be taught faster than he can learn. The speed of the horseman must be limited by the power of his horse. Every man that has ever undertaken to instruct others can tell what slow advances he has been able to make, and how much patience it requires to recall vagrant inattention, to stimulate sluggish indifference, and to rectify absurd misapprehension.