Scholarship requires single-minded devotion; you can't be of two minds.
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Literary and rare in everyday speech. You'll see it in classical-flavored writing about scholarship, faith, or craftsmanship. Compare with the more common 专心 ('concentrated') and 专注 ('focused').
memory
笃 carries the sense of 'sincere, earnest, deep' (also in 笃信 'firmly believe'). 专 = 'concentrated, exclusive'. Earnestness + exclusivity = total devotion.
