Full Idea
Whitehead envisages the building blocks of reality not as substances at all, but as 'actual occasions' - processual units rather than 'things' of some sort.
Gist of Idea
Whitehead's building blocks are unit processes, called 'actual occasions'
Source
report of Alfred North Whitehead (Process and Reality [1929]) by Nicholas Rescher - Process Metaphysics 01.10
A Reaction
Rescher isn't a fan of this version of process philosophy, presumably because if processes are divided into units, they would be hard to distinguish from substances. Aristotle is credited with the active substance view. Do processes need building blocks?
Book Reference
Rescher,Nicholas: 'Process Metaphysics' [SUNY 1996], p.20