"One thing we ought to have clear in our minds at the outset of a modelling endeavour is whether we are intent on describing a portion of "reality" (some human enterprise) or a data processing activity."
Gregersen, Heidi; Jensen, Christian S. (1999). "Temporal Entity-Relationship models—a survey". IEEE Transactions on Knowledge and Data Engineering. 11 (3): 464–497. CiteSeerX10.1.1.1.2497. doi:10.1109/69.774104.
Gregersen, Heidi; Jensen, Christian S. (1999). "Temporal Entity-Relationship models—a survey". IEEE Transactions on Knowledge and Data Engineering. 11 (3): 464–497. CiteSeerX10.1.1.1.2497. doi:10.1109/69.774104.