From WordNet (r) 1.6 [wn]:

  idealism
       n 1: the doctrine that ideas are the only reality
       2: impracticality by virtue of thinking of things in their
          ideal form rather than as they really are
       3: elevated ideals or conduct; the quality of believing that
          ideals should be pursued [syn: {high-mindedness}, {noble-mindedness}]

Used here both in the first and third senses. At the core of knowledge evolution is the ideal that making sure ideas see the light of day, are discovered by others and are subject to criticism (expression/transmission/evaluation). The people who support this ideal are idealistic dreamers: they have set their sights on a far off goal of changing how people think.