Thoreau: 7 January 1852

“It appears to me that at a very early age—the mind of man—perhaps at the same time with his body, ceases to be elastic. His intellectual power becomes something defined —& limited. He does not think as expansively as he would stretch himself in his growing days—What was flexible sap hardens into heartwood and there is no further change. In the season of youth methinks man is capable of intellectual effort 8c performance which surpasses all rules 8c bounds—As the youth lays out his whole strength without fear or prudence 8c does not feel his limits. It is the transition from poetry to prose. The young man can run 8c leap—he has not learned exactly how far—he knows no limits—The grown man does not exceed his daily labor. He has no strength to waste.
17 January 1852, Journal 4: 265-66