"Once Zhuangzi dreamt he was a butterfly, a butterfly flitting and fluttering around, happy with himself and doing as he pleased. He didn't know he was Zhuangzi. Suddenly he woke up and there he was, solid and unmistakable Zhuangzi. But he didn't know if he was Zhuangzi who had dreamt he was a butterfly, or a butterfly dreaming he was Zhuangzi." (quoted from wikipedia)

The generally accepted idea is that waking life is real, and dreaming is an illusion, but how can one know for certain that this is true? Could not dreaming be real, and waking the illusion? Or could they are both be real, or both illusory?

Discuss.