Fluid intelligence isn’t the only kind—there is also crystallized intelligence. This is defined as the ability to use a stock of knowledge learned in the past. Think once again about the metaphor of a vast library. But this time, instead of regretting how slow the librarian is, marvel at the size of the book collection your librarian is wandering around in, and the fact that he knows where to find a book, even if it takes him a while. Crystallized intelligence, relying as it does on a stock of knowledge, tends to increase with age through one’s forties, fifties, and sixties—and does not
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