Characterized by refinement and niceness in drawing
   distinctions; nicely discriminating; -- said of persons; as, a subtle
   logician; refined; tenuous; sinuous; insinuating; hence, penetrative or
   pervasive; -- said of the mind; its faculties, or its operations; as, a
   subtle intellect; a subtle imagination; a subtle process of thought;
   also, difficult of apprehension; elusive.