Artist Tino Sehgal: 'Our culture is hung up on and overemphasises what can be derived from material objects. I think this is something quite new, over the past 200 or 300 years – that life has become about accumulating material wealth. The 21st century is not about accumulating material wealth like the 20th century. It's already eroding. I'm not against material things – I just don't work with them.'