The internet isn't exactly known for considered, civilised discussion, but publishing site Medium has managed to buck that trend.

It's a site used by the likes of President Obama, Bono, Bill Gates and Hillary Clinton when they have something serious to get off their chests.

The BBC's North America technology reporter Dave Lee met Medium's founder, Evan Williams - who also co-founded Twitter.

Read more: Inside Medium: An attempt to bring civility to the internet