Why is a negative x a negative = a positive?

Polotskaia, E. (2009). Why is a negative x a negative = a positive? Possible Benefits of Interdisciplinarity. In B. Sriraman & V. Freiman (Eds.), Interdisciplinarity for the twenty-first century. (pp. 257–278). Charlotte, NC: Information Age Pub.

Abstract: Considering the concept as a mental mechanism, I propose a simple model of concept functionality. From this model follow some criteria of the concept construction conditions. Using this model, I analyse different didactic possibilities of development of the negative numbers understanding in school students. Then, I propose a Physical Science context that nicely meets discussed criteria. Some preliminary testing indicates that this context could possibly be used to start the negative numbers concept development as early as in primary school.