qua actual entities, with a single exception, God. For example, for Aristotle, the actual entities were the substances, such as Socrates (a particular citizen of Athens) and Bucephalus (a particular horse belonging to Alexander the Great). Besides Aristotle's ontology of substances, another example of an ontology that posits actual entities is in Leibniz' monads, said to be 'windowless'. Whitehead's actual entities For Whitehead, the actual entities exist as the only foundational elements of reality, the ultimately existing facts of the world. Nothing "either in fact or in efficacy" underlies or lies beyond the actual entities; rather they underlie all reality. The actual entities are of two kinds, temporal and atemporal. With one exception, all actual entities for Whitehead are temporal and are occasions of experience (which are not to be confused with consciousness, or with mere subjectivity). This 'actual entity' idea is most distinctly characteristic of the metaphysics of Process and Reality, and requires of the newly approaching reader a philosophically unprejudiced approach. An entity that people commonly think of as a simple concrete object, or that Aristotle would think of as a substance – a human being included – is in this ontology considered to be a composite of indefinitely many occasions of experience. The one exceptional actual entity is at once temporal and atemporal: God. He is objectively immortal, as well as being immanent in the world. He is objectified in each temporal actual entity; but He is not an eternal object. Whitehead uses the term 'actual occasion' to refer only to purely temporal actual entities, those other than God. The occasions of experience are of four grades. The first comprises processes in a physical vacuum such as the propagation of an electromagnetic wave or gravitational influence across empty space. The occasions of experience of the second grade involve just inanimate matter. The occasions of experience of the third grade involve living organisms. Occasions of experience of the fourth grade involve experience in the mode of presentational immediacy, which means more or less what are often called the qualia of subjective experience. So far as we know, experience in the mode of presentational immediacy occurs in only more evolved animals. That some occasions of experience involve experience in the mode of presentational immediacy is the one and only reason why Whitehead makes the occasions of experience his actual entities; for the actual entities must be of the ultimately