An everyday example of a constraint is: a definition.
   The definition can be fixed. Even the very word itself: define.

   Directly from Latin here:

   definire "to limit, determine, explain," from de- "completely"
   (see de-) + finire "to bound, limit," from finis "boundary, end"

   boundaries, constraints. A set of markers that tell someone,
   "This is defined as depression", are constraints on the system
   and the actions/behaviors/thoughts/whatever is measurable that
   is occuring acively *within* the boundaries of that definition,
   is considered "depression". The definition fixes the
   measurement; but the measurement does not fix the active mind
   from thinking/emoting - and potentially being more than its
   definition.

   Rather self-fulfilling in a way.