Domain Expertise is an absolute necessity to come up with Rigorous Specifications and in particular; for Safety Critical Systems.
If they can be done by a single person (whatever be his role name) all the more better but usually for Safety Critical (and highly specialized) Systems that is not possible and hence you need two (or more) people.
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The problem is domain experts aren't actually that good specifications either.
Do you know who is trained to be good at spec'ing software? Software engineers.
You have missed the point.
Domain Expertise is an absolute necessity to come up with Rigorous Specifications and in particular; for Safety Critical Systems.
If they can be done by a single person (whatever be his role name) all the more better but usually for Safety Critical (and highly specialized) Systems that is not possible and hence you need two (or more) people.