Definition.com
Thinking (n.) is kept alive by being better pleased with oneself than with one's lot.
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Thinking (n.) is hard to begin and impossible to stop: once a thinker, always a thinker.
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Thinking (n.) worked for him only in the open air: the forest at his door was to be his study.
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Thinking (n.) is rare by nature: in both sexes there are only two classes, those who think and those who do not, and the difference is almost entirely one of education.