verb (v)

• mark fine distinctions and subtleties, as among words(verb.cognition)

  source: wordnet30
• make more subtle or refined(verb.change)

  Synonym:
  rarefy, sublimate
  

  source: wordnet30
• make (senses) more keen(verb.change)

  Synonym:
  subtilise
  

  source: wordnet30
• To make thin or fine; to make less gross or coarse.(verb)

  source: webster1913
• To refine in argument; to make very nice distinctions.(verb)

  source: webster1913