Learning activity develops in junior schoolchildren the reflexive ability making it possible to differentiate known and unknown and, using hypotheses concerning unknown, to address one's own and partner's (peer's, teacher's) action in joint solving of new problems. Ability to ask for information, to criticize actions and opinions, to be independent in evaluation and self-evaluation, tendency to solve any problem by discutable means are behavioral manifestations of junior schoolchildren's as learning activity subjects' reflexive development. Learning activity does not develop the abilities which are not reflexive - credulity, imitation, idle fantasies. These abilities are developed in other kinds of activity. A teacher planning school education must combine all the kinds of activity to avoid one-sided development.