presentational immediacy. We may say that the brain has a material and a mental aspect, all three being abstractions from their indefinitely many constitutive occasions of experience, which are actual entities. Time, causality, and process Inherent in each actual entity is its respective dimension of time. Potentially, each Whiteheadean occasion of experience is causally consequential on every other occasion of experience that precedes it in time, and has as its causal consequences every other occasion of experience that follows it in time; thus it has been said that Whitehead's occasions of experience are 'all window', in contrast to Leibniz's 'windowless' monads. In time defined relative to it, each occasion of experience is causally influenced by prior occasions of experiences, and causally influences future occasions of experience. An occasion of experience consists of a process of prehending other occasions of experience, reacting to them. This is the process in process philosophy. Such process is never deterministic. Consequently, free will is essential and inherent to the universe. The causal outcomes obey the usual well-respected rule that the causes precede the effects in time. Some pairs of processes cannot be connected by cause-and-effect relations, and they are said to be spatially separated. This is in perfect agreement with the viewpoint of the Einstein theory of special relativity and with the Minkowski geometry of spacetime. It is clear that Whitehead respected these ideas, as may be seen for example in his 1919 book An Enquiry concerning the Principles of Natural Knowledge as well as in Process and Reality. In this view, time is relative to an inertial reference frame, different reference frames defining different versions of time. Atomicity The actual entities, the occasions of experience, are logically atomic in the sense that an occasion of experience cannot be cut and separated into two other occasions of experience. This kind of logical atomicity is perfectly compatible with indefinitely many spatio-temporal overlaps of occasions of experience. One can explain this kind of 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 each of indefinitely many other actual entities. 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