of experience, is logically atomic in the sense that it cannot be cut and separated into two other occasions of experience. This kind of logical atomicity is perfectly compatible with indefinitely many spatiotemporal overlaps of occasions of experience. One can explain this atomicity by saying that an occasion of experience has an internal causal structure that could not be reproduced in each of the two complementary sections into which it might be cut. Nevertheless, an actual entity can completely contain indefinitely many other actual entities. Whitehead's theory of extension concerns the spatio-temporal features of his occasions of experience. Fundamental to both Newtonian and to quantum theoretical mechanics is the concept of velocity. The measurement of a velocity requires a finite spatiotemporal extent. Because it has no finite spatiotemporal extent, a single point of Minkowski space cannot be an occasion of experience, but is an abstraction from an infinite set of overlapping or contained occasions of experience, as explained in Process and Reality. Though the occasions of experience are atomic, they are not necessarily separate in extension, spatiotemporally, from one another. Indefinitely many occasions of experience can overlap in Minkowski space. An example of a nexus of temporally overlapping occasions of experience is what Whitehead calls an enduring physical object, which corresponds closely with an Aristotelian substance. An enduring physical object temporally has an earliest and a last member. Every member (apart from the earliest) is a causal consequence of the earliest member of the nexus, and every member (apart from the last) of such a nexus is a causal antecedent of the last. There are indefinitely many other causal antecedents and consequences of the enduring physical object, which overlap, but are not members, of the nexus. No member of the nexus is spatially separate from any other member. Within the nexus are indefinitely many continuous streams of overlapping nexūs, each stream including the earliest and the last member of the enduring physical object. Thus an enduring physical object, like an Aristotelian substance, undergoes changes and adventures during the course of its existence. Another aspect of the atomicity of occasions of experience is that they do not change. An actual entity is what it is. An occasion of experience can be described as a process of change, but is itself unchangeable. Whitehead's abstractions Whitehead's abstractions are conceptual entities that are abstracted from or derived from and founded upon his actual entities.