Constitutional Law 101.

You would think that the Prime Minister of all people would be up to speed on constitutional law, but it appears not.  He is seeking to get a Bill passed which would guarantee that there would be a referendum on our continuing membership of the EU before the end of 2017 - that is, well into the next Parliament.  This completely ignores one of the fundamental principles of constitutional law, that "Parliament cannot bind its successor".  This means that any decision taken by this Parliament can be countermanded by the next one - or indeed by this one if they later change their minds.  So it can be seen as an expression of intent - of wishful thinking perhaps - but nothing more.