  This paper extends the work of a previous paper [arXiv:1208.2611] on the flow
of time, to consider the origin of the arrow of time. It proposes that a `past
condition' cascades down from cosmological to micro scales, being realized in
many microstructures and setting the arrow of time at the quantum level by
top-down causation. This physics arrow of time then propagates up, through
underlying emergence of higher level structures, to geology, astronomy,
engineering, and biology. The appropriate space-time picture to view all this
is an emergent block universe (`EBU'), that recognizes the way the present is
different from both the past and the future. This essential difference is the
ultimate reason the arrow of time has to be the way it is.
