> The circular specification problem > The only way to know exactly what software we want to build …

Software specification is circular because what we write down as a requirement is going to depend heavily on what is feasible to implement. But feasibility of implementation is often not clear until you actually try to implement the thing. In fact, I would go so far as to say that, if in the design phase the precise structure of every single piece of what you are implementing, and the scope and feasibility of all alternatives, is known and crystal clear, then the project is trivial and probably does not need a design phase - you might as well just write the code. The article makes this point as well: "a complete, abstract specification is, in general, at least as much work as creating the software itself"

There's only so much requirements can do to prepare one for a project. At some point there's diminishing returns on time spent spec'ing something out. And of course requirements can change, so some amount of flexibility in being able to pivot while work is in progress is ideal. Building software is usually more of a process than a plan. This is in contrast of traditional engineering where you absolutely need a rigid plan to build something like a bridge or a building. Of course reality is messy and no plan will be perfect

To expand on what others have said: If the spec isn't the size of the software, then it leaves out details. But "everybody knows what we mean" - until they try to implement it. Then they find out part of what wasn't specified. "Part of", because there's the other part, which wasn't specified, and they just assumed there was only one possible answer, and so they didn't even realize that there was a gap in the spec there. But there is, and if someone, somewhere, makes the opposite assumption, the gap matters.

" ...a new (often anticipatable) requirement seems to appear... " And often not, particularly when you are building a new system. And if you are replacing an existing system, the requirements that you discover are usually, "it must work exactly the same way as the old system."