"But I say unto you..."?

The Lord Jesus uses this phrase numerous times in the Sermon on the Mount.  To me it's indicative that he is laying the groundwork for a new dispensation, moving from that of the law (hence the numerous references to Moses), to that of the kingdom, which is referred to numerous times in the passage as well. Some have referred to the Sermon on the Mount as the constitution of the coming kingdom, and I agree with that.