> "Knowing what to build" is fine for technical people who are building a simple web based tool which they'll use themselves. However, this doesn't generalise to building software for complex industries.

Who said anything about software?

> As an example, a few years ago I was advising a company who wanted to automate the calculation of tax. You would have thought that this was easy,

No, I wouldn't, because I know a little about taxes. A technical tax person would know even more and understand a lot of the complexity.

> but in fact it's the most complex thing ever and you need that non-technical specialist domain knowledge.

Why do you persist in saying that domain knowledge is necessarily non-technical and that technical people can't have it?

Both are wrong.