  The overall objective of Requirements Engineering is to specify, in a
systematic way, a system that satisfies the expectations of its stakeholders.
Despite tremendous effort in the field, recent studies demonstrate this is
objective is not always achieved. In this paper, we discuss one particularly
challenging factor to Requirements Engineering projects, namely the change of
requirements. We proposes a rough discussion of how learning and time explain
requirements changes, how it can be introduced as a key variable in the
formulation of the Requirements Engineering Problem, and how this induces costs
for a requirements engineering project. This leads to a new discipline of
requirements economics.
