This chapter illustrates how educators and policymakers can benefit from Sir Karl Popper's ideas as they implement the two-year Teacher Education Program in Ontario, Canada, and how his ideas may stand on firmer footing than the inductionist approach, which currently prevails. Bereiter draws a clear distinction between learning and knowledge building by defining learning as acquiring knowledge and knowledge building as helping others to know. Moreover, Bereiter claims that teachers need to move beyond the knowledge acquisition stage to build knowledge. The activity of understanding is the same as that found in all problem-solving. Like all intellectual activities, it consists of subjective 'World' processes. According to Popper, individuals engage with their surroundings on three levels. The first is the physical world, or the world of physical states; the second is the mental world, or the world of mental states; and the third world is the world of ideas in the objective sense.