Scrolling: document up or viewport down?

My first graphical user interface was a Sun 3 running SunView. That's a classic WIMP interface bordering on antique. I was already a programmer so it's hard for me to separate the machine from the user interface. The machine's model of scrolling consists of a viewport window, a scrollbar and data to display (frequently another window). This is the model-view-controller paradigm invented for Smalltalk. It's easy to see the scrollbar thumb as being the position of the viewport over the data, so scrolling down means moving the thumb and viewport down the data and holding the data still.

My three year old son using a touch interface has a completely different mental model. The machine still uses the same model-view-controller paradigm, except the UI consists of dragging the document up rather than moving a scrollbar thumb down. What I call scrolling down, my son calls up. His model is more intuitive and simpler in every way.

This is really exciting! After 30 years of stagnation, user interfaces are finally changing.