What is and what is known about what is are two concepts which are often entangled, especially in quantum mechanics (puns intentional). This is one of the subjects which I cover in my upcoming book, incidentally, but which I’d like to expand in a second edition. (I’m assuming it’ll be a best seller.)
Here’s a lump of clay. It exists; it is actual. But also existing, in a sense, is the potential for that lump to be an ashtray (attention young readers: the ashtray was a device to catch cigarette ashes. I miss people smoking.) The ashtray is in the lump; it exists in potentia.
Now at the very top is a being of pure act, which is to say, God; a being (being itself!) in which there is absolutely no potency, i.e. no potential to change (we covered this in our Summa Contra Gentiles series). At the bottom (I use these positional terms metaphorically) is prime matter, stuff which is pure potential and which contains no actuality; you can’t measure or collect a bucketful of it. Everything else (including you) is in between, containing a mixture of act and potency. Some objects are weighted more heavily towards act, others towards potency (but there is no claim to form a numerical measure of this “weight”).
All this is preliminary to follow the paper “Werner Heisenberg and Thomas Aquinas on Natural Indeterminism” in New Blackfriars by Ignacio Silva.
Ed Feser in his must-read Scholastic Metaphysics has collected apt quotations from Heisenberg in describing quantum reality. Here’s one (p. 126, from H’s Physics and Philosophy; ellipsis in quotation):
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 reality just in the middle between possibility and reality. (p. 15)
And, more to our point, this:
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)
Regular readers recognize this is what I insist is the proper definition of “subjective” probability: people having different premises for the same proposition of interest. (The standard Bayesian view I reject because probability is not “feelings.”)
Finally to Silva, who spends most of his time showing Aquinas anticipated quantum mechanics, as it were, at least as far as causes go. He says, for instance, “Thomas explicitly rejects a rigid determinism in nature, i.e. the position that whenever there is a cause a certain determinate effect necessarily follows”, etc. The reasons for Aquinas advances for this are not quite quantum mechanical, however, but have more to do with unaccounted forces and the presence of multiple causes. Granting all this still does not tell us what actuality (God? our non-material intellects, at least sometimes?) reduces the potentia of a QM wave to actuality.
I can’t do the whole paper, but here’s Silva on what’s pertinent to us:
As we descend the degrees of being, the corresponding reduction in actuality correlates with an increase in potentiality, down to the forms of the elements, which are the closest to prime matter, pure potency…
Greater or lesser actuality comes from the participated esse, received by the essence. Essences which are closer to matter would be those that would have lesser actuality, thus, greater potentiality. The farther the substance is from pure actuality, the greater its potentiality, and thus the greater the possibility of an ineffective action. That is why Thomas says that there are three main spheres of action within reality: 1) that being which is only act [God], operates always without defect; 2) that which is only potency, pure matter, needs an act to actualise it; and 3) that which is a mixture of act and potency, every natural being, which acts perfectly most of the times…
We find, then, in every natural being a passive indetermination, which is essentially an imperfection or — more accurately and absolutely speaking – a lack of perfection in relation to the whole of being. According to the hierarchy of being postulated above, we can say that natural things, as they are farther from Pure Act, they participate less in act: they are less determinate.
Now there are some who argue the QM wave exists, i.e. it is ontic (see Ringbauer et al.). This existence is not specified, at least as far as I can tell, in terms of a mixture of actuality and potentiality; instead, the wave just exists. That means all the potential end states exist actually in some “superposition”. But then, since the wave-as-actuality “collapses” to one point, it must exist at least in potential to these end states. And it also exists in potentia in the sense the entire wave can exist potentially in other localities.
This is why it makes much more sense to go with Heisenberg (and Aquinas) and say the wave exists, like we do, as a mixture of actuality and potentiality. In what portions, I have no idea. A “pure” wave, interacting with nothing, would exist as pure potential, i.e. would be prime matter; though I doubt the QM wave is pure potential. But here is where my ignorance is most glaring. I don’t know enough to see what parts of the QM wave are actual and what part potential. Silva says “it does not sound very implausible to affirm that quantum mechanics is working and describing natural things which are great in potency and low in act.”
Silva also thinks QM waves “cannot be pure potency, as Heisenberg claimed, because they would be primer matter itself, which needs to exist under some kind of formality…For sub-atomic particles to be considered in potency, they need to be under a formal determination, and thus some degree of actuality”
Anyway, the main point (for us) is that our knowledge of the potential end states, which are not actual, is not the wave, even if we have full knowledge of the wave, but is a function of the wave (its “squared modulus”) and other premises. In other words—the exciting conclusion—probability is not actual, nor is it even a potential. It is fully epistemic. Probability doesn’t exist, even if wave functions do as full actualities. Silva (with his spellings):
In particular, matter in these quantum systems can take unpredictable forms, which are only predicted probabilistically by the wave-function included in the Schrödinger equation. Although matter is open to the reception of new forms, it cannot receive any form. The system described by the Schrödinger equation could only receive those forms included probabilistically in that equation.
In other words, not only does probability not exist, epistemically all probability is conditional on the premises fix, here by Schrödinger’s equation. (A QM wave for an electron is potentially, say, a particle and not potentially a Buick.)