The two clearest examples of actual occasions are, thus, a momentary experience, whether of a human being or of some other animal, and a quantum of energy. By giving them the same name, Whitehead calls attention to what, with all their differences, they have in common. First, we may point out what they are not. They are not “matter” in the Greek or the modern sense. That is, they are not passive recipients of form or action. They act to constitute themselves as what they become.
Second, they become what they become out of a given world. What they are is largely a function of what other things are. In the case of the quantum, it is what it is largely because of the quantum field in which it occurs. In the case of a moment of human experience, it is what it is largely because of the character and content of antecedent human experiences and the neuronal events in the brain. But the principle of uncertainty in the case of the quanta and our awareness of an element of choosing indicate that, however much it is limited by the past, an actual occasion does decide exactly what it will become. Also, in both instances, what it becomes informs future actual occasions. An actual occasion is acted on, it acts in its own synthesizing of its data, and it acts in future occasions. The word “actual” is rich in meaning.
Cobb Jr, John B. Whitehead Word Book: A Glossary with Alphabetical Index to Technical Terms in Process and Reality (Toward Ecological Civilization Book 8) . Process Century Press. Kindle Edition.