Simeon Schlicht demonstrates through empirical studies that children acquire the concepts of quantity and number simultaneously and in relation to each other. In his research involving children aged 3 to 4 years, the author uses play situations, which he interprets according to the rules of interaction analysis. The results contrast with the prevailing scientific view that children first learn the concept of quantity and then build upon it to understand the concept of number. By linking numbers to real situations, arithmetic for children takes on more of the character of a natural science rather than a formal mathematical theory. The author combines approaches from cognitive psychology, educational sciences, and the philosophy of science, thereby creating instructive cross-connections. The findings provide insights for educators and teachers.