  The traditional understanding of stakeholders requirements is that they
express desirable relationships among phenomena in the relevant environment.
Historically, software engineering research has tended to focus more on the
problems of modeling requirements and deriving specifications given
requirements, and much less on the meaning of a requirement itself. I introduce
new concepts that elucidate the meaning of requirements, namely, the designated
set and the falsifiability of requirements.
  By relying on these concepts, I (i) show that the adaptive requirements
approaches, which constitute a lively and growing field in RE, are
fundamentally flawed, (ii) give a sufficient characterization of vague
requirements, and (iii) make the connection between requirements modeling and
the Zave and Jackson sense of engineering. I support my claims with examples
and an extensive discussion of the related literature. Finally, I show how
adaptation can be framed in terms of Zave and Jackson's ontology.
