Abstract: The experimental study showed: 1. Causality thinking in pre-school children is shown in the course of experiment; 2. Children's active attitude (or mental set) and eagerness to find out the solution of the problem is an essential condition for the development of causality thinking; 3. There are several stages in the transformation of actions into thoughts, and their characteristics vary with different age levels; 4. Reversibility in thinking is revealed in the discovery of the conservation of quantity of liquid in children of 6—7 years group but not in children of 3—4 years group.

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