To understand the development of knowledge we must start with an idea which seems central to me – the idea of an operation. Knowledge is not a copy of reality. To know an object, to know an event, is not simply to look at it and make a mental copy or image of it. To know an object is to act on it. To know is to modify, to transform the object, and to understand the process of this transformation, and as a consequence to understand the way the object is constructed. An operation is thus the essence of knowledge; it is an interiorized action which modifies the object of knowledge.

jean piaget

Piaget, J. (1964). Part I: Cognitive development in children: Piaget. Development and learning. Journal of Research in Science Teaching, 2(3), 176–186.