general kind. Consequently, it is inessential that an occasion of experience have an aspect in the mode of presentational immediacy; occasions in the grades one, two, and three lack that aspect. The highest grade of experience "is to be identified with the canalized importance of free conceptual functionings". There is no mind-matter duality in this ontology, because "mind" is simply seen as an abstraction from an occasion of experience which has also a material aspect, which is of course simply another abstraction from it; thus the mental and the material aspects are abstractions from one and the same concrete occasion of experience. The brain is part of the body, both being abstractions of a kind known as persistent physical objects, neither being actual entities. Though not recognised by Aristotle, there is biological evidence, written about by Galen, that the human brain is an essential seat of human experience in the mode of 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. Inherent in each actual entity is its respective dimension of time. Potentially, each 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; 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. 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. Time in this view is relative to an inertial reference frame, different reference frames defining different versions of time. The actual entity, the occasion