Werner Heisenberg Quote
[The probability wave] meant a tendency for something. It was a quantitative version of the old concept of "Potentia" in Aristotelian philosophy. It introduced something standing in the middle between the idea of an event and the actual event, a strange kind of physical reality just in the middle between possibility and reality.
Physics and Philosophy: The Revolution in Modern Science, Chapter II (p. 41), Harper & Row, Publishers. 1958