Another crucial assumption regarding the nature of the learner concerns the level and source of motivation for learning. According to Von Glasersfeld, sustaining motivation to learn is strongly dependent on the learner's confidence in his or her potential for learning. These feelings of competence and belief in potential to solve new problems, are derived from first-hand experience of mastery of problems in the past and are much more powerful than any external acknowledgment and motivation. This links up with Vygotsky's "zone of proximal development" where learners are challenged in close proximity to, yet slightly above, their current level of development. By experiencing the successful completion of challenging tasks, learners gain confidence and motivation to embark on more complex challenges.