The quality of having a sharp, unconfused mind; the ability to think and reason clearly without mental fog or confusion.
From 'clearheaded' plus '-ness' (Old English suffix for abstract nouns). This triple-suffix word represents the full nominalization of the root concept: clear → clearheaded → clearheadedness.
This word is a champion of English suffixing—it stacks suffixes like building blocks to turn a simple concept ('clear') into an abstract quality that philosophers and psychologists can debate and measure.
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