How does the spook interact with solid matter? How does an ethereal nothing respond to flashes, pokes and beeps and get arms and legs to move?
Ah yes, the interaction problem. Is that still around? Apparently so.
By "spook," Pinker evidently means the "soul." But the use of the term betrays a profound misunderstanding of what traditional philosophers have meant by that term. He seems to imagine the soul as a sort of substance independent of the body that somehow inhabits the body and "interacts" with it. The old Cartesian ghost-in-the-machine; now exacerbated by post-modern software-hardware analogies. (Such analogies might be fruitful, but we must always keep in mind that they merely reflect post-modern fascination with computers. The human being (or any part of nature) is under no obligation to behave, let alone to be, anything like a computer.
Chastek has some fun pointing out the absurdity:
In one way, it’s hard to see how this problem arises: souls are forms, and so if this were really a problem, then there would also be an interaction problem for basketballs: How does a sphere interact with solid rubber? How does a Euclidean solid respond to the molecular structure of hydrocarbons? What an untenable dualism!”
Recall that a "substance" is a hylomorphic union of matter with form. Prime matter itself is formless and is Pure Potency. As such, it is not "in act" or "actual," a point missed by Heisenberg when he speculated that matter-energy was the prime matter. What makes matter actually a substance is that it is some form of matter. That is, a form is united with matter in "an act of existence." All matter encountered in actuality has a particular form: a brown cow, a down quark, whatever. The medieval slogan was "Every thing is some thing." Similarly, we do not encounter forms off by themselves in a Platonic world of pure abstract forms. The medieval slogan was "No white without a white thing."
Now we can conceive and consider forms independently of their matter. That's what reason does. It is the essence of science. So we can think about whiteness without necessarily considering white light or snow or polar bears. We can think about triangularity without considering an actual physical triangle.
Hylomorphic union is a dualism of a sort. Just like "geometric figure" and "three-sided" is a dualism that makes a triangle exist; or "mass of rubber" and "sphere" is a dualism that makes a basketball exist. But it is not the "substance dualism" of Descartes and the Moderns. There is still only one substance: a triangle, a basketball, a human being. There is an interaction problem only when there are two things to interact. When you see a basketball, Chastek asks, do you see two things: a basketball and its roundness?
It turns out that soul-as-Stephen-Pinker understands it is a silly and mythical entity that owes its existence to Stephen Pinker (or any of us) being deceived by false imagination. First, we turn forms into things simply speaking; when in fact forms are certain things only with qualification that are necessary to explain what we call things simply speaking. If the verbiage there is too dense, just ask yourself if the roundness of a basketball is a thing without any qualification. When you see a basketball, do you say that you see two things, a basketball and its roundness?
All in all, another indication of how the "traditional problems" of philosophy are really artifacts created by the Moderns mucking up a perfectly fine metaphysics.
And to some extent by abandoning a perfectly fine language. In Latin, the word translated as "soul" is anima, which means simply "alive." Thus, the question whether people, poodles, or petunias have souls is simply to ask whether they are alive. Next question. (OK. The next question would be whether they are alive in the same way. Ans. No. But we digress.)
The soul as form of the body is not merely the shape of the body, but includes all its powers and functions. A live petunia does not differ from a dead petunia in its matter. both consist of the same parts, the same chemical elements. They differ in that the dead petunia is just a bag of chemicals and will behave that way from that point on. But the live petunia is not just a bag of chemicals, but a bag of chemicals "in process" (Process is a nice contemporary word) and as such it behaves like a petunia: growing and developing, maintaining its metabolism, its homeostasis with the environment, reproducing, and so on. It is, as it were, "in motion."
A side issue of all this: If all substances encountered in actuality are unions of matter and form and, down in the substrate as it were, is Prime Matter, which is Pure Potency, symmetry demands that at the other end of the scale there be some sort of Prime Form [or Soul], which is Pure Act. Just as Prime Matter is matter without form, this Prime Form would be form without matter. Just as Prime Matter would provide matter for all stuff, the Prime Form would give form to all stuff; that is, it would set stuff in motion. What that "prime mover" might be is a head-scratcher, all right.