The imagination of a boy is healthy, and the mature imagination of a man is healthy; but there is a space of life in between, in which the soul is in a ferment, the character undecided, the way of life uncertain, the ambition thick-sighted; thence proceeds mawkishness.
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Gender-inclusive version:

The imagination of a boy or girl is healthy, and the mature imagination of a man or woman is healthy; but there is a space of life in between, in which the soul is in a ferment, the character undecided, the way of life uncertain, the ambition thick-sighted; thence proceeds mawkishness.