From a new biography of Alexander I by Sergey Tzvetkov:

He could think independently and even make conclusions based on what he had heard; [the empress] Catherine had once said that he seems to educate himself. Without knowing coercion, he learned to obey, and he did it without a sense of duty but with eagerness. The Empress remarked to [former Russian ambassador to Germany] Grimm, that when [Alexander's brother] Konstantin had a cold and was forbidden to approach the window, a line was drawn through the room lest he forget. Nevertheless the small child kept crossing the line; then Alexander approached him and said:

--Brother, when I am told not to go past here, I, in order to remember this, draw another line in my mind behind the one meant for me, and when, due to my forgetfulness, I pass my own line, I remember that I should not cross the next one.