We all like to effect something, but so far as the love of power is concerned we do not care what we effect. Broadly speaking, the more difficult the achievement the more it pleases us. Men like fly-fishing, because it is difficult ; they will not shoot a bird sitting, because it is easy. ... A child, at first, delights in walking, then in running, then in jumping and climbing. What we can do easily no longer gives us a sense of power ; it is the newly-acquired skill, or the skill about which we are doubtful, that gives us the thrill of success.
Source: On Education, especially in early childhood, 1926, by Bertrand Russell
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