Thursday, September 17, 2009

Heisenberg on act and potency

What’s that? You say you’re itching for more quotes from popular science books written by major early twentieth-century quantum physicists, which tend to support a classical metaphysical picture of the world? Well OK then, friend, you’ve got it. Today let’s take a look at Werner Heisenberg’s Physics and Philosophy.

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)

And again:

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.


  1. I started to read D. Q. McInerny's Metaphysics and in it he said both the actual and the potential are real and as an example of the latter he said the apple which would be on his desk two years from now. If the A-T position explains change as the actualization of potentiality of an actually existing thing (subject of change) it makes sense. But if it also means non-existing things are real because they can be conceived or imagined as existing - that doesn't make sense: they are "real" only as something in the mind. In other words, what makes sense to me is that there is no reality or potentiality to non-existing things because there is no subject of change. If I agree that an apple tree may have an apple which I could place on my desk 2 years from now the fact remains that that apple is not real. Is McInerny's example wrong or am I to accept that what is conceivable or imaginable is real independently of my mind?

  2. "What’s that? You say you’re itching for more quotes from popular science books written by major early twentieth-century quantum physicists, which tend to support a classical metaphysical picture of the world?"

    Hey, it's like you read my mind. "Spooky", as Einstein would undoubtedly say.

  3. Ed,

    You write that "[Heisenberg] proposes, for example, that quantum theory calls for a revision of the law of the excluded middle."

    Of course, that law can not be revised except that it be abolished. And to abolish that law would return us to the paradoxes of Parmeniades and Heraclitus, which Aristotle had solved by his conception of potency and act, in the light of the self-evident principle of the excluded middle.

    So you say that Heisenberg defends the ideas of potency and act, but he also denies the principle upon which those ideas depend.

    I would guess that Heisenberg probably ended up identifying potency with the (formerly) excluded middle.

  4. I've always found Heisenberg's position on this interesting, because his approach has an interesting twist. One of the things he recognized is that in order to investigate a problem, you have to be able to formulate good questions; so while he doesn't accept the work of ancient philosophers as explanatory, he repeatedly insists, in a number of places, that it was excellent question-building. That's how I interpret his take on Aristotle here: act and potency won't necessarily appear as such in any scientific explanation of any natural event, but they still do an excellent job providing a way to frame the overarching general questions of science those scientific explanations will be trying to answer in ways that fit the particular phenomena to be explained. Thus he allows for some looseness of fit (I think the energy as Aristotelian matter is an example of this: energy is what in the scientific explanation of certain phenomena in physics has a function that makes it correspond to the potentia in the general philosophical questions about all the phenomena in the world). Somewhere William Wallace, O.P., has published an exchange of letters with Heisenberg on the energy/materia question, which sheds a lot of light on his views; at least, it was Wallace if I'm remembering correctly.

    It really is the case that most of the major European physicists of Heisenberg's generation, and of the generation immediately before, had truly solid philosophical educations. Even if it was mostly Kant and Mach, it was enough to guarantee that they usually had genuinely interesting and intelligent things to say about philosophical subjects. Whenever you go back and read them, it's almost always striking how informed and thoughtful they were on an immense range of topics.

  5. Really enjoying the Heisenberg posts, Professor. Great stuff!


  6. Oops, forgot to write Schrodinger as well~


  7. Ed,

    A question about formal/final causes. You give the opium "dormative power" example in TLS to illustrate an aristotilean take on potencies. Would that mean that, say, regarding a given chemical as corrosive, or magnetic, or an acid, etc, would be (perhaps subconsciously) offering up an Aristotilean explanation?

    Also, and this is going to be a weird, nerdy question: What about the periodic table of the elements? How would such a table/classification fit into an Aristotilean/Aquinas perspective?

  8. "For example, don't too easily trust philosophical musing about quantum physics emanating from the first generation of physicists who were inventing quantum mechanics. They were just trying to figure out what the blazes was going on, and inescapably they went down many blind alleys in the process. That's how it goes."

    from Taner Edis, weighing in on this.

  9. It seems it's less a problem with "philosophical musings" than with the wrong, undesirable kind of philosophical musings. I have yet to find a person who desires philosophy and metaphysics as nonsense or unreliable, and who doesn't also engage in it (usually, quite a lot of it) themselves. Pay no mind to the philosophical ponderings of Heisenberg and company - they were just trying to make sense of their new theories! Now, Darwin's philosophical ponderings about evolution.. those are different.

  10. Anon:

    Taner Edis, who knows a bit about quantum physics, becomes irritated when folks like Feser and Michael Egnor try to use quantum physics to shore their magical belief system.

    I don't think Edis denies that philosophy is a useful or legitimate discipline.

  11. And Edward Feser, who knows quite a bit about philosophy, likely becomes irritated when folks like Edis butcher philosophy by not knowing the line between physics and metaphysics. And in this particular case it's more Edis dismissing Heisenberg than anything else.

    Tell Edis people, even non-scientist non-philosophers, are going to speculate about the connections between science, metaphysics, and otherwise. Also, to get used to it, because his particular view isn't sacrosanct.

  12. Anon,

    "Edis people" are aware.

    I for one quite enjoy metaphysical speculation.

  13. Fr. William A. Wallace, O.P.'s The Modeling of Nature claims that matter-energy is as close as we have gotten to primary matter; in fact he uses it almost synonymously with primary matter.

  14. Has any Thomist yet written a paper on quantum superposition and the law of non-contradiction?

  15. Read THOMISM AND THE QUANTUM ENIGMA by William A. Wallace, O.P., a book review in The Thomist of Wolfgang Smith's book of the same title.

    @Anonymous: Perhaps the resources here might help.