The Transmission Account of Causation
There are two types of transmission accounts; those that take transmission to occur between events, and those that take transmission to occur between what Salmon would call a causal process, i.e. persistent objects. If we take the event-based accounts first, then Kistler presents the core idea in what he calls a reduction statement:
- (T) Two events a and b are causally related in the sense that one is a cause of the other if and only if there exists a conserved quantity Q of which a particular amount P is transmitted between a and b.
- (Kistler 1998: 1)
I find this unsatisfying for three interrelated reasons. First, because it offers no criteria to distinguish causal and non-causal instances of transmission.
Second, because it doesn’t say what will bring about a transmission of a conserved quantity in the causal instances. Third, because we don’t have a generalisable account of the transmission of conserved quantities; it doesn’t work for all the cases.
The first problem is that conservation laws entail that any quantity present in any Kim-style event will be transmitted to the next event e„ whether or not anything causal occurs between ( and e,. A uniformly moving body will through any given temporal interval constitute a succession of distinct Kim-style events, between which any conserved quantity possessed by any event en will be transmitted to a subsequent event eo. The same holds for two successive states of any arbitrarily chosen material system, regardless of what happens within the system.
The causal process version has the advantage of narrowing things down a little more, since it specifies that the transmission is not just between two distinct events, but between two causal processes, i.e. persistent particulars, when they interact. Dowe (2009: 219) offers the clearest statement of this view, by combining criteria previously put forward by himself and Salmon:
CQ1. A causal interaction is an intersection of world lines that involves exchange of a conserved quantity.
CQ2. A causal process is a world line of an object that possesses a conserved quantity. (Dowe 1995: 323)
Definition 1. A causal interaction is an intersection of world-lines that involves exchange of a conserved quantity.
Definition 2. A causal process is a world-line of an object that transmits a nonzero amount of a conserved quantity at each moment of its history (each spacetime point of its trajectory).
Definition 3. A process transmits a conserved quantity between A and B (A / B) if it possesses [a fixed amount of] this quantity at A and at B and at every stage of the process between A and B without any interactions in the open interval (A, B) that involve an exchange of that particular conserved quantity. (Salmon 1997: 462, 468)
Kistler—underestimating how uncharitable critics like me are willing to be—obviously assumes in his reduction statement that everyone realises that the statement is only meant to apply to what happens in interactions between particulars, but Dowe makes this explicit. Dowe’s account takes care of the first and second problem. Causal transmissions occur in events involving interactions between two distinct particulars, and it is the interaction between the two particulars that triggers the transmission. It also brings us closer to the picture offered by physics, which indeed depicts conserved quantities as being transmitted between particles. Physics even postulates virtual particles as carriers for any quantity that appears to be transmitted between spatially discrete entities.
However, the third problem remains. The assumption is that the direction of causation follows the flow of the transmitted quantity—really, that it follows the flow of energy—and this flow is assumed to explain the nature of causation. Why did the ball in motion make the ball at rest move when they collided? Because a quantity of motion was transmitted from one to the other. My problem with this explanation is that it does not generalise to all the relevant cases and will eventually suggest that we need to postulate something more to explain the manner in which the quantities flow, in order to get an account that generalises; we need to postulate that interacting objects always exert an equal and opposite influence on each other, and that this explains the resulting transmission (or distribution) of the conserved quantities. Let me explain this further.
The process version of the transmission account seems to make pretty good intuitive sense only as long as we stick to the standard examples. That is, examples of an apparently active (or more active) thing acting on another more passive thing which suffers a change: a billiard ball in motion encounters another at rest, and changes the state of motion of the latter from rest to motion (Hume 1748: § 36); a leaden ball dropped onto a pillow produces a hollow in the pillow (Kant 1787: A203); a locomotive pulls a truck (Taylor 1973: 35); a baseball is hit by a bat to strike and break a window (Salmon 1980: 50). All these examples fit the standard view, according to which interactions involve an active object influencing a passive object, or, as Aristotle claimed: ‘whenever the potential active and the potentially affected items are associated in conditions propitious to the potentiality, the former must of necessity act and the latter must of necessity be affected’ (Metaphysics: 9,5; 264). Consider instead two identical billiard balls moving at the same speed, but in opposite directions, that collide head on. They will compress each other and then bounce back in opposite directions, both moving (roughly) at the same speed again but now in opposite directions from before. Which ball transmitted a quantity to the other? Did both transmit to the other, or was any quantity transmitted at all? Perhaps both balls merely changed the direction of their respective quantities without transmitting anything between them? If we are confined to merely describing the manner in which quantities go, we have no explanation, and we have no universally valid account of the direction of causation. To my mind, and which will be elaborated on in Chapter 4, what is missing here is some account of how interacting particulars exert an influence on each other and which explains why the conserved quantities are distributed/changed the way they are when particulars interact. The mere exchange of quantities doesn’t answer that question.
Traditionally, it is the concept of ‘force’ that has explained how interacting particulars influence each other. However, in the friction between reductionistic and anti-reductionist accounts, the concept of force has been just as controversial as the notion of power. Empiricists like Heinrich Hertz wanted to get rid of the concept—to him it represented one of the occult and mysterious entities we postulate only through an invalid inference from observed changes to some imagined invisible cause to those changes—and instead wanted to describe interactions between material entities merely in terms of the changes in the state of motion that they can be observed to suffer. Accordingly, he suggested the second law of motion should not be understood as saying that the force is proportional in magnitude to the object’s mass times the acceleration it suffers, but as stating an identity of force and change in state of motion. It really meant that the third law of motion (F1on, = ~F,oni) could just as well be expressed by a reconstructed two-way second law; mt x at = m, x a,. In plain English, when two material systems interact, the observed change in the state of motion of the first (mt x a}) is always equal to the observed change in the state of motion of the second fm, x a2). Why should we accept a reduction of this kind? The empiricist answer is that we must otherwise appeal to the occult notion of force.
Now, to my mind, the proponents of the transmission account should not really have any qualms about the concept of force, in so far as they seek to ground their view in the notions already in use in the natural sciences. However, to accept it, they have to accept that there is a more fundamental feature to physical interactions than the transmission of conserved quantities. This they do not do, and therefore end up in want of an explanation of why conserved quantities are transmitted in the way they in fact are.
