Common Causes and The Direction of Causation†
Brad Weslake‡

Abstract

Is the common cause principle merely one of a set of useful heuristics for discovering causal relations, or is it rather a piece of heavy duty metaphysics, capable of grounding the direction of causation itself?  Since the principle was introduced in Reichenbach’s groundbreaking work The Direction of Time (1956), there have been a series of attempts to pursue the latter program—to take the probabilistic relationships constitutive of the principle of the common cause and use them to ground the direction of causation.  These attempts have not all explicitly appealed to the principle as originally formulated; it has also appeared in the guise of independence conditions (Ehring, 1982; Papineau, 1985a, 1985b, 1993; Hausman, 1998), counterfactual overdetermination (Lewis, 1979, 1986), and, in the causal modelling literature, as the causal markov condition (Pearl, 2000; Spirtes et al, 2000).  Of course, those developing the causal modelling framework do not take themselves to be providing an account of the asymmetry of causation.  For a start, they do not standardly take themselves to be pursuing reductive projects in the first place.  Moreover, if you ask a causal modeller about the place of the direction of causation in their work, they will tell you that temporal orientation is simply assumed.  But—science for the scientists, philosophy for the philosophers—there have been a number of philosophers who take it that the causal modelling framework provides reductive grounds for the direction of causation after all (see Papineau, 1993; Field, 2003).  In the present paper, I argue that no such reduction of causal asymmetry in terms of the common cause principle, or any of the related notions, is possible.

I begin by motivating the project, in terms that make it of independent interest to both causal modellers and philosophers of causation.  I highlight the methodological benefits of making clear the situations under which attributions of causation are warranted—by asking, for any particular causal claim, which relevant facts can be varied so as to preserve the claim, and which facts can be varied so as to lead one to revise the claim.  In particular, I argue that getting clear about the situations in which one would be prepared to accept the existence of backwards causation provides a means by which the concept of causation, so interwoven with other temporal and modal concepts, can be isolated and clarified.  More important though, for the purposes of the present paper, is the converse point—that taking some set of characteristic features of causal relations, and imagining them to exist in the reverse temporal direction, allows the exploration of whether these features are constitutive of the causal relation or just defeasible heuristics for the causal relation.  It is the application of this method to the principle of the common cause with which the remainder of the paper is concerned.

In the following part of the paper, I set out the original common cause principle and the way in which the central feature of causation it identifies is captured by the various formulations it has been given.  I then describe some standard objections to the principle, and the standard means by which it has been defended against these objections.  The dialectic here allows the extraction of some important lessons regarding the status of the principle—most important are the role of sampling and the selection of populations, and the role of the causal network in which a particular causal relationship is situated.  Relating these to the issue of the direction of causation, I show that there are important issues here regarding where to place causation in the context of a reductive project.

Following this discussion, I separate two distinct issues regarding the placement of causation.  Firstly, there is the role of the causal network in fixing the orientation of any particular causal claim.  This strategy for fixing the direction of causation also originates with Reichenbach, and in this section I criticise the general strategy with specific reference to Dowe (2000), who provides the most recent elaboration.  Secondly, there is the issue of where causation lies on the microscopic to macroscopic scale.  The tensions here for the reductive attitude to causal models is brought out nicely by the famous billiard ball example of Salmon (1980, p. 223).  I set up the problem by first describing the reasoning from the time-symmetric determination of fundamental physical laws to the absence of causal asymmetry in fundamental physics, which leads Field (2003) to restrict the causal relation to those variables which we find salient at the macroscopic level.  I then show that the standard response to problems for the principle of the common cause (and probabilistic causation more generally), which have been framed in terms of such salient variables, has been to appeal to a more complete description of the situation in terms of microscopic variables.  I take Hausman and Woodward (1999) as a case in point, and show how in a deterministic system there will be a massive underdetermination of causal model by the relations between the microscopic variables, such that any time slice of the system will, on some model, fulfill the causal markov condition for any other time slice of the system.  The common cause principle, then, faces pressure from both above and below—and it is not clear that it is left with any space to inhabit.

In the final section of the paper, I make good on the opening methodological point, by describing a straightforward case of the temporal inversion of the probabilistic basis for the principle of the common cause, where we are not led to suppose that the direction of causation has been reversed.  The case relies on the Einstein, Podolsky and Rosen (EPR) correlations in quantum mechanics, but can be generalised to make the central point—the common cause principle cannot ground the direction of causation.  In closing, I offer some thoughts on the lessons which ought be drawn from the preceding discussion.

References
Ehring, Douglas. 1982. “Causal Asymmetry”, in The Journal of Philosophy, Vol. 79, No. 12, December 1982, pp. 761–774.
Hausman, Daniel M. 1998. Causal Asymmetries, Cambridge University Press, Cambridge, 1998.
Hausman, Daniel M. and Woodward, James. 1999. “Independence, Invariance, and the Causal Markov Condition”, in British Journal for the Philosophy of Science, Vol. 50, No. 4, 1999, pp. 521–583.
Lewis, David. 1979. “Counterfactual Dependence and Time’s Arrow”, in Noûs, Vol. 13, No. 4, November 1979, pp. 455–476.
————. 1986. “Postscripts to “Counterfactual Dependence and Time’s Arrow””, in his Philosophical Papers, Vol. II, Oxford University Press, New York, 1986. pp. 52–66.
Papineau, David. 1985a. “Causal Asymmetry”, in British Journal for the Philosophy of Science, Vol. 36, 1985, pp. 273–289.
————. 1985b. “Probabilities And Causes”, in Journal of Philosophy, Vol. 82, No. 2, February 1985, pp. 57–73.
————. 1993. “Can We Reduce Causal Direction to Probabilities?”, in Hull, David; Forbes, Micky and Okruhlik, Kathleen (Eds). 1993. Proceedings of the Biennial Meetings of the Philosophy of Science Association 1992, Philosophy of Science Association, East Lansing MI, 1993, Vol. 2, pp. 238–252.
Pearl, Judea. 2000. Causality. Cambridge University Press, Cambridge, 2000.
Reichenbach, Hans. 1956. The Direction of Time, University of California Press, Berkeley, 1956.
Salmon, Wesley. 1980. “Probabilistic Causality”, in Pacific Philosophical Quarterly, Vol. 61, 1980, pp. 50–74.  Reprinted in his Causality and Explanation, Oxford University Press, Oxford, 1998, pp. 208–232.
Spirtes, Peter; Glymour, Clark; and Scheines, Richard. 2000. Causation, Prediction and Search, 2nd Ed, MIT Press, Cambridge MA, 2000.