A striking coincidence is much like a small drama: The Causal Figment

A striking coincidence is much like a small drama: the participants arrive exactly on cue, wear the right makeup, know their lines, and produce results that are significant or trivial, amusing or, sometimes, awe-inspiring. The problem is that no scriptwriter, no director, no stage manager, and no collusion on the part of the actors are involved in the performance; it unfolds, without reason but with perfect order, as though by magic.

Coincidences are baffling because they seem to represent order arising by chance: they resemble the results of an orderly causal process, but they do not have a causal connection that fits our experience. For example, the beetle that flies into the psychiatrist's consulting room just as a patient is recounting a dream in which such an insect enters her room has no discernible connection with the patient. It could not have known how to enter the room on cue. Furthermore, the patient who dreams of this liberating encounter has no way of knowing that it will occur or any means of ensuring that her response will be the one predicted by the dream.

The problem with coincidences is that they violate our notions of cause and effect. But supposing our notions of causality are wrong?

In 1739 the Scottish philosopher David Hume published A Treatise of Human Nature, an analytical rejection of the commonly established ideas of causation. In philosophical terms, his arguments have never been fully rebutted; in scientific terms, much of what he maintained has been justified.

Since Greek philosophers first turned their attention to causality in the fifth-century b.c., it had been almost universally accepted that everything that has a beginning must be caused by something else. Hume rejected this. On the contrary, he maintained, it is not certain that every object which begins to exist must owe its existence to a cause. To believe, said Hume, that every being must be preceded by a cause is no more valid than believing that because every husband must have a wife, every man must therefore be married.

Hume aimed to show that the traditional starting point for theories of causation is incapable of proof; he was not, of course, trying to prove its reverse or any other position. All we can justly say of causality is that what we take to be a cause always precedes what we take to be its effect, and that there is always continuity between the two. Beyond this, he said, nothing could be claimed, and the view that a necessary connection exists between a cause and its effect is nothing more than a habit of mind.

The Scottish philosopher David Hume

The Scottish philosopher David Hume held that the idea of a causal relationship between two events occurring in sequence is nothing more than a habit of mind.

For example, while watching a game of billiards, we confidently expect that when one ball strikes another, the ball that has been struck will move, and we, therefore, persuade ourselves that there is a connection between the motion of the first ball and the motion of the second—between cause and effect. Such an idea, however, is not based on logic or observation, Hume said. All we observe is that contact—contiguity— occurs; the rest is an assumption. Our expectation that a stationary ball will move in a predictable way when struck by another ball may well be correct in most cases, but this is not a certainty. The momentum and inertia of the two balls must be considered—too little momentum or too much inertia, and the effect will not be what we expect. The materials from which the balls are made must also be taken into account, and so must their soundness—is one of the balls apt to shatter rather than move? We must also consider the shape of the balls, the nature of the surface on which they lie, and the stability of the situation in which the event takes place. Among all these variables and many more, we look in vain for an identifiable principle connecting cause and effect; and since we look in vain for it, we are under no compulsion to assert its existence or to accede to such assertions.

Although Hume's arguments may appear to fly in the face of common sense, they have to some extent been vindicated by 20th-century physics. At the subatomic level, ideas of predictability (which should pertain, at least theoretically, if causal connections could be found or even theoretically established) have been replaced by those of statistical probability.

Established ideas of causality have also come under fire at a macroscopic level, particularly among evolutionary biologists. For example, how can we describe the evolution of the reptilian egg in terms of cause and effect? Evolutionary theory holds that changes in organisms occur as the result of random genetic mutations; if one of these changes confers an advantage that allows the organism to produce more offspring, the change is likely to be inherited by the offspring and may eventually become normal for the species. But when we look at the reptilian egg (or the mammalian eye or any number of other features and organs), we see that numerous events must have occurred simultaneously for the development to succeed. The shell, for instance, had to be impermeable and strong enough to protect the embryo. But unless the embryo had at the same time developed some means of liberating itself from the shell, this durable egg would have become a tomb. In addition, the embryo had to develop a means of absorbing nutrition while in the egg. But unless it had also developed some means of storing its own waste products safely, it would soon have created a poisonous environment.

Each of these developments—the durable shell, egg tooth, and so on—had to arise, according to evolutionary theory, as the result of random mutations. But between the mutations that produced the shell and those that produced the egg tooth there could have been no connection (they arose at random), nor between those concerning nutrition and waste disposal. And if there were no such connections, how was the whole process orchestrated? From this point of view, the reptilian egg must be seen as appearing without causal benefit and as representing the culmination of a series of wildly improbable coincidences.

David Hume was well aware that his view of causality would be hard for people to accept when he ascribed the difficulty to the force of mental habits that condition our outlook. If he was right—if we expect causal connections—we have only ourselves to blame (or congratulate) when we find coincidences a tantalizing and titillating affront to the commonsense view we hold of the world.

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