I’d like to show here that interactionist substance dualism (ISD) is a scientific hypothesis in the sense that typical versions of it are testable through observation. That is to say that we can describe conditions of evidence under which it would be reasonable for scientists to accept ISD.
ISD is the view that immaterial minds and material bodies interact with each other causally; the mind can control certain movements of the body, and the body can influence the mind. Descartes held that mind and body were intimately entangled in causal interaction. ISD is meant to explain why when a pattern of light enters the eye (which is just a material thing) your mind (a supposedly immaterial thing) has a conscious experience. It would also explain why when you try to reach for something, your hand (being just a material thing) is compelled to reach. For Descartes, a key distinction between the material and the immaterial is that the former is extended in space, while the latter is not.
(Worth noting before moving on: 1. How exactly a material thing could be affected by an immaterial thing is the notorious classical problem of interaction advanced by Princess Elizabeth of Bohemia, and Descartes is not commonly thought to have resolved it. 2. Whether Descartes ultimately or actually held ISD is a question of scholarship I won’t pursue here. 3. That modern science agrees with Descartes at least on the proposition that not everything is a material (i.e., space-filling and obstructive) thing is, I think, interesting and worth keeping in mind.)
Now, let’s consider generally what would count as good evidence for causal interaction between two things, only one of which—the material thing—could be publicly observed. If that material thing acts in principled ways that could not be explained by its structure or what we know of the immediate material environment, then we would have good reason to suppose that it is receiving an undetected signal from elsewhere. For example, today even non-scientists among us have good evidence that something (the electromagnetic field) interacts with material things such as radios, remote control toys, and smartphones because these things behave in ways we cannot explain by local material considerations alone.
Imagine scientists of Descartes’ time—17th-century natural philosophers—coming across some of the remotely-controlled toys or drones of our time. (We hide behind the bushes and send these drones about, operating them and sensing their environment from afar.) After catching and “dissecting” some of these drones, the natural philosophers could learn a lot about the roles of the units inside: Removing the battery would make its function obvious; Cutting the circuit running from the camera would result in a drone that acted as if it were blind; Careful analysis of the circuit board and modifications to it would allow these scientists to gain a robust understanding of the way our drones work. These 17th-century investigators would have reason to postulate the presence of otherwise undetected “animating influences” running from the battery along the circuits and to the motors. Eventually, a thorough enough examination would give these natural philosophers an understanding of the complete relevant causal structure of these drones, but they would be left with some very important questions:
What makes that antenna thing change states so that it sends the signals it sends through the rest of the system, controlling the motor and gears in such a way that the drone behaves as it does? And when this other thing (the transmitter) changes states, so what? It doesn’t seem to effect the rest of the system in any material way. And yet changes in that thing appear to correlate in interesting intelligent ways with later changes in the antenna thing. But how could they? There appears to be a material gap in this otherwise perfectly intelligible causal structure.
Careful scrutiny of what was around the antenna would reveal nothing that interacted with it causally in a manner sufficient to change its state so that it sent the signals it sent. From the point of view of these 17th century investigators, it would appear either that the antenna just changed states sui generis, or that it was receiving as of yet undetected signals. That is, either two pieces of metal just behave in accordance with each other as if they are causally connected in an undeniably intelligent way even though they are not, or they are causally connected in that way even though the connection itself has not been observed. I would argue that their evidence favors the latter hypothesis: the drones are in causal contact with something else undetected via these two points (antenna and transmitter). And of course, were the natural philosophers to make this assumption, they would be correct.
Bringing it back to today, we can conceive of neuroscientists being in a situation analogous to that of the 17th-century natural philosophers, but with respect to human bodies instead of drones. Were we ever to discover a point in the brain that changed state inexplicably in this kind of way—a causal gap, we would have good reason to suppose ISD.
Now, to my knowledge we have not discovered any such causal gaps in the brain (though I can imagine advocates of QM theories of consciousness arguing that we have, perhaps at the level of microtubules), and if so, we do not have this reason in favor or ISD. In fact, if we gain more evidence for a causally closed physical description of the central nervous system, we may have more reason to regard ISD as increasingly less likely.
Will our empirical investigations of the brain come to a causal gap conducive to ISD? I’m not holding my breath, but I do think these considerations about undetected signals show how the question of ISD is a matter of empirical investigation.