Like many scientists of his generation (and unlike contemporary scientists like Richard Dawkins and Jerry Coyne), Heisenberg knew something about philosophy and its history, and took its problems seriously. In particular, he recognized that empirical science requires for its intelligibility a sound philosophy of nature (or metaphysics, as we might say today, though the other term is preferable – the philosophy of nature concerns the preconditions of there being an intelligible natural world, while the concerns of metaphysics are more general than that). Moreover, he saw that a return to certain classical philosophical notions was essential to making sense of modern physics.
Of course, it can hardly be maintained that Heisenberg subscribed in any wholesale way to a classical metaphysical picture of the world; he proposes, for example, that quantum theory calls for a revision of the law of the excluded middle. But he did at least tentatively endorse something like the fundamental notion of Aristotelian-Thomistic (A-T) metaphysics – the famous distinction between act and potency.
Regarding the “statistical expectation” quantum theory associates with the behavior of an atom, Heisenberg says:
One might perhaps call it an objective tendency or possibility, a “potentia” in the sense of Aristotelian philosophy. In fact, I believe that the language actually used by physicists when they speak about atomic events produces in their minds similar notions as the concept “potentia.” So the physicists have gradually become accustomed to considering the electronic orbits, etc., not as reality but rather as a kind of “potentia.” (pp. 154-5 in the 2007 Harper Perennial Modern Classics edition)
The probability wave of Bohr, Kramers, Slater… 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. (p. 15)
And yet again:
The probability function combines objective and subjective elements. It contains statements about possibilities or better tendencies (“potentia” in Aristotelian philosophy), and these statements are completely objective, they do not depend on any observer; and it contains statements about our knowledge of the system, which of course are subjective in so far as they may be different for different observers. (p. 27)
Discussing, more generally, the relationship between matter and energy in modern physics, Heisenberg says:
If we compare this situation with the Aristotelian concepts of matter and form, we can say that the matter of Aristotle, which is mere “potentia,” should be compared to our concept of energy, which gets into “actuality” by means of the form, when the elementary particle is created. (p. 134)
Now an A-T philosopher would want to clarify and qualify these claims. In the first two quotes, Heisenberg contrasts “potentia” with “reality.” What A-T says, though – and what Heisenberg himself clearly means, given the context – is not that potentials are not in any sense real, but rather that qua merely potential they have not been actualized. (Act and potency are in fact both real, but they are different kinds of reality. Part of the point of the distinction is to note that Parmenides’ notorious absolute distinction between being and non-being is too crude: There is, within the realm of being, a difference between the actuality of a thing and its potentials, and the latter are not to be assimilated to sheer non-existence.)
Furthermore, energy in the modern sense wouldn’t count as matter in the Aristotelian sense if what Heisenberg means by that is “prime matter,” viz. matter without any form whatsoever; for energy in the modern sense, given that it has a specific physical description, has form. (It might instead be, though, that what Heisenberg means to suggest is only that energy is the most fundamental kind of in-formed matter.)
In any event, it is clear that what Heisenberg is defending is a core thesis of A-T philosophy of nature, namely that we cannot make sense of the physical world behaving as it does without attributing to its basic components inherent powers which point beyond themselves to certain (often as yet unrealized) ends – a thesis that, as I have noted before, contemporary writers like Ellis, Cartwright, Molnar, and other “new essentialist” philosophers of science are starting to rediscover.