Which episode of Family Guy had the exchange between Brian and Stewie on the topic of the Flat Tax?

I was wondering the same thing! It's the one where Stewie meets his future self, and Old Stewie shows Young Stewie Rupert the teddy bear. Rupert is missing a leg or something, and Young Stewie asks what happened, then flashes back to the time when he and Brian were debating the flat tax, and Brian started eating Rupert.