“When a man reaches the point where he concludes that he ‘knows’ his subject, he decides, consciously or subconsciously, that he has nothing more to learn, and he promptly begins to lose what he does know; when he becomes aware that he has ‘grown up’, he has reached a stage where he has already begun to stultify those potentialities for growth which once were his, and which might have been his to the end. Boredom, monotony, and discontent follow swift upon the establishment of this condition.”
(Taken from “Constructive Conscious Control of the Individual” published 1923 – p193)