The Collaborative International Dictionary of English v.0.48:

Subtlety \Sub"tle*ty\, n.; pl. Subtleties. [OE. sotelte,
   sutilte, OF. sotillete, L. subtilitas. See Subtle, and cf.
   Subtility.]
   1. The quality or state of being subtle, or sly; cunning;
      craftiness; artfulness.
      [1913 Webster]

            The fox which lives by subtlety.      --Shak.
      [1913 Webster]

   2. Nice discernment with delicacy of mental action; nicety of
      discrimination.
      [1913 Webster]

   3. Something that is sly, crafty, or delusive.
      [1913 Webster]

            Unlearned in the world's false subtleties. --Shak.
      [1913 Webster]