In the study reasons are presented for the dependence of the phenomena of non-retention of length, quantity and weight in Piaget tasks on the lack of differentiation in the reflection of different object properties. The data are reviewed on the successful forming of the capacity to fulfil retention tasks with the non-retenting children. It is shown that behind the very different, on the sight, techniques of such forming there is always a process of developing more articulate and differentiated reflection of different object properties. It is concluded that viewing mental activities as analytico-synthetical and using modern cognitive psychology concepts the age called by Piaget a period of concrete operations formation must be qualified as a period of forming and cognitive differentiation of well-developed mental representations of perceived properties of physical objects.