As the western world winds its way through the 40th anniversaries of 1968 and the ‘summer of love’, _Steve Platt_ looks back at the role of cannabis in the ‘counter culture’ and how people on both sides of the political and cultural divide believed that a hardy psychoactive plant could change the world. He wonders how it could ever have aroused such passions – both for and against its use – and asks why it’s still illegal