The sermon on the Mount - what actually is it?

So Matthew presents us with a block of teaching from Jesus as if it were a sermon.
ALERT! ALERT! Heresy coming!
Does that mean that I think Jesus didn't *actually* preach it all as one sermon?
Well, no, not necessarily. I think Jesus had a body of teaching that he delivered many times over, varying what he was saying each day, depending on the context, how well the crowd was listening, what people said to him in response, and so on.
What he didn't do, I'm pretty certain, was only say these things once, one one single occasion, just as Matthew wrote it all down.
Why do I think that? Well, most obviously because Luke's gospel also contains a lot of this material, but subtly different and in something of a different order, and says that Jesus said it on another occasion. (Not up a mountain, but on a "level place.") Mark has bits of this stuff as well, but not all collected together in one lump.
I also think this way because I'm a preacher myself, and I can't imagine preparing all this wonderful material and only delivering it once. I'm always recycling ideas, saying the same things over and over again in different ways, chopping and changing each week, depending on my audience and their reactions. My long suffering congregation know only too well that I have my favourite sayings, which crop up again and again.

So why does Matthew give it to us like this? One reason is it suits his purpose. He wants to present Jesus as a new Moses. New and improved. Just as Moses went up a mountain and came down with some instructions from God for the people to follow, so Jesus teaches from a mountain and gives people new and improved instructions to follow.
Several times over, Jesus says "You have heard it was said..." and then quotes Moses, then goes on to say, "well I say to you..." and gives a new commandment, a harder command.
New and improved. Right.
And here's another reason. It's good to have it all together. Like a manifesto, or indeed like a sermon, it's good to have the whole of Jesus' key teaching laid out like this, ripe to remember. Ready to land on our ears and filter into our hearts.
So I'm looking forward to committing it to memory. I don't think it will be too hard. There are a number of features that I expect will help. It has a rhythm and a cadence that is easy to assimilate. It works by constructing opposing pairs: "Do this ... don't do that." "People used to think this, but I'm telling you that." It has vivid pictures: salt, light, planks sticking out of your eye and houses built on sand falling with a crash. Jesus didn't say it like that to help me memorise it, he said it like that to prevent his hearers forgetting what he said, and so imitating the foolish builder themselves.
With God's help, I pray that my learning will make me wise.