On this week's red lines, we're looking at the challenges unionism is facing as the conversation about constitutional change here continues.

Many unionists don't want to engage in the debate.

Some are even critical of the fact that it's happening at all.

But there are those who feel unionism has to confront the reality of the changing political landscape on the island of Ireland.

And we're going to hear from one of them in a moment.

He's the historian, Aaron Edwards, and he has a new book out later this month.

He grew up in Rathcule just outside Belfast in a family steeped in orange and unionist culture.

He studied at Queen's University, and he now lectures in defence and international affairs

at the Royal Military Academy Sandhurst.

Aaron Edwards sets himself the task of explaining what unionism is.

He considers what the lessons of the last century might be,

and he suggests what a more inclusive forward-thinking unionism might look like.

We'll hear from the man himself in just a moment.

But first let's talk to our former political editor, Mark Devonport. Mark, we know there are those

within unionism who say very simply, we should not engage in a discussion about Irish unification

because to do so is to be complicit in bringing about our own demise. And that seems to be a pretty widely

held view at the moment. You can see the logic of it, which is that if they buy into the

nationalist project, all the talk about what the future island might look like, then they are,

in effect, writing their own obituary.

But we've had disagreements within unionism.

I can remember, of course, there was that very heated conversation between Sammy Wilson and Peter Robinson,

when Peter Robinson said it was as well to always have an insurance policy

...