There are two equal and opposite pathologies of the common ground. One is the refusal to share any, to maintain clear divisions between yourself and those you disagree with. The other is to pretend or imagine people have more in common with you than they do, latching on to any kind of similarity as evidence of sameness.

To find the right mean between the two we need to be honest about what our similarities and differences are – but, more fundamentally, be clear as to what the point of trying to establish common ground is. Do we seek shared territory because we want the world to be a lovely, friendly place where deep disagreements are rare and no one holds stupid, ignorant or deluded beliefs? Whatever our psychological motivations, I think there are two sets of good...