Do you have to believe what you write in your Ph.D. thesis for it to be valid?

So it was my PhD viva yesterday. It lasted 2 hours and 20 mins and was exhausting! I had (as is usual in the UK system) two examiners; one internal (a member of my University) and an external (from another institute). The exam wasn't a structured one and we meandered around our thesis with lots of discussion and questions as we went. It was very tiring overall.