Heisenberg on possibility

In his 1958 lecture, ‘Planck’s discovery and the philosophical problems of atomic physics’,1 Heisenberg traces the quantum physics doctrine of probability waves back to Aristotle:

the essence of matter [concerns] (…) the Greek philosophers’ old question of how it is possible to reduce to simple principles the motley and manifold phenomena surrounding matter and thus make [those phenomena]2 intelligible.3

the work of Bohr, [Hans] Kramers and [John Clarke] Slater contained the decisive concept, that the laws of nature determine not the occurrence of an event, but the probability that an event will take place, and that the probability must be related to a wave field that obeys a mathematically formulable wave equation.
This was a decisive step away from classical physics; basically a concept that played an important part in Aristotle’s philosophy was used. The probability waves of Bohr, Kramers and Slater can be interpreted as a quantitative formulation of the concept of “possibility” in Aristotle’s philosophy, Greek dynamis (potentia in the later Latin version).4 The concept that events are not determined in a peremptory manner, but that the possibility or “tendency” for an event to take place has a kind of reality — a certain intermediate layer of reality, halfway between the massive reality of matter and the intellectual reality of the idea or the image — this concept plays a decisive role in Aristotle’s philosophy. In modern quantum theory this concept takes on a new form; it is formulated quantitatively as probability and subjected to mathematically expressible laws of nature. The laws of nature formulated in mathematical terms no longer determine the phenomena themselves, but the possibility of happening, the probability that something will happen. (16-17)

  1. Die Plancksche Entdeckung und die philosophischen Probleme der Atomphysik’, lecture from the 13th conference of the Rencontres Internationales de Genève, September 4, 1958. Translation in On Modern Physics, 1962, 9-28.
  2. Translation: ‘them’.
  3. The circularity implicated in this passage is highly important to note. The Aristotelian tradition maintained that the answer to the question, ‘how it is possible to reduce to simple principles the motley and manifold phenomena surrounding matter and thus make them intelligible’, was to appeal to the range of possibilities underlying those phenomena. But how is it possible to access possibilities without already having done so? without having activated that possibility from the range of available possibilities that first gives access to that range?
  4. The translation reads: “interpreted as a quantitative formulation of the concept of dynamis, “possibility”, or in the later Latin version, potentia, in Aristotle’s philosophy.”