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When it Comes to the Crunch

Trevor Emmott probes David Hume’s unreasonable view of cause and effect.

How many causes have you witnessed today? Even if you are reading this at breakfast, the list is long. Moments ago, the turning of a tap made water flow. Outside, you see trees shaken by the wind. From the instant you awoke, you have been the passive observer of countless causal relationships – and an active participant in many others.

Small wonder that causation has intrigued philosophers – the most notable being David Hume [1711-76]. He asks how we acquire the concept of a ‘necessary connection’ between cause and effect. We feel that the cause somehow makes the effect happen. Given the cause, the effect must follow. But how do we get that idea – and is it justifiable? Hume reaches a startling verdict.

He quickly concludes that necessity is not something we detect just by looking. As Hume puts it: “we are never able … to discover any power or necessary connection; any quality which binds the effect to the cause and renders the one an infallible consequence of the other.” All we perceive is a mere succession of events. “The impulse of one billiard-ball is attended with motion in the second. This is the whole that appears to the outward senses.”

It is repeated observation of the conjunction of the same events which creates the idea of a necessary connection between them. Then, Hume says, “the mind is carried by habit, upon the appearance of one event, to expect its usual attendant.” The idea is the product of what Hume calls the ‘imagination’. Had Hume lived two centuries later, he might have talked instead of conditioning. If a bell is rung whenever a dog is fed, soon the bell alone makes the dog’s mouth water. Like dogs, we have become conditioned. We have often seen a moving body make contact with a still one and immediately observed a movement of the latter. Now on contemplating such impacts, we just cannot help expecting movement. And, by some confused thinking process, that psychological link between ideas is mistaken for a real connection in the physical world between events themselves.

Reason plays no part in generating this idea of necessary connection. Nor can the idea be justified afterwards by rational argument. This, the most controversial aspect of Hume’s theory, is what I wish to explore.

Imagine you are on a railway platform. On the track beside you is a stationary freight wagon. In the distance you see a locomotive slowly approaching along the same track. Eventually, the locomotive makes gentle contact with the wagon and begins shunting it along. Clearly, this is a causal relationship – but can anything here justify talk of necessary connection?

At first, as you watch, you may feel sure that the wagon will move when contact is made. On reflection, though, you must admit that there are other conceivable outcomes. For example, at the very point of contact the locomotive might stop dead. Movement of the wagon can be no more than probable and never certain. Despite that, necessity is to be discovered here. To find it, what may seem an odd question must first be answered. It is this: can two objects be in the same place at the same time?

If they could, life would be so different. Take driving, for instance. If two cars could simultaneously occupy the very same stretch of road, motorists would lose all fear of collisions. We would speed across junctions without looking, yet in perfect safety (and no worries about the police). What’s more, if a parking place was already occupied by another vehicle, we could drive right in regardless.

Sadly, such a paradise is unattainable and inconceivable. You cannot visualise a red car and a green car in the same place at the same time. The two images may alternate rapidly in your mind, but at any single moment you do not have an idea of two cars at all, but of one only. It is simply impossible for two objects to occupy the same place at the same time. This self-evident truth is immensely important.

To understand why, return to the railway example. When you start watching the approaching locomotive, what I propose to call the ‘basic facts’ can be stated as follows. The locomotive is moving forward and the wagon is stationary. That statement remains true for a while. But it must become false as soon as the locomotive makes contact with the wagon. Then, the locomotive is about to move onto the same length of track where the wagon is standing. In other words, the locomotive is threatening to enter the space already occupied by the wagon. If (for even an instant longer) the locomotive keeps moving forward and the wagon stays where it is, then the two objects will be in the same place at the same time. And that is impossible.

So the basic facts must change at the moment of contact. Either the locomotive must stop following its present course; or the wagon must cease occupying its current position. You cannot predict specifically how those basic facts will change. The infinite possibilities range from the plausible to the ludicrous. The wagon might be shunted along. The locomotive might sprout wings and fly away. One event can however be predicted with absolute certainty – and that is the change in the basic facts.

This is the crucial point. Contact between locomotive and wagon must be followed by a change in the basic facts. These two events – contact and change – are logically linked. Change is (in Hume’s terms) the ‘infallible consequence’ of contact. Between contact and change there is a genuine necessary connection.

At the same time, contact and change are surely related as cause and effect. It is contact between the two objects which causes the basic facts to change. So these are two events between which there is both a causal relationship and a necessary connection. And this we discover, not from repeated experience of conjunction (as Hume would claim), but from rational reflection upon a single case.

Perhaps you feel that change is a peculiar kind of event to treat as an effect. You would accept however that (say) heating metal causes a change in its colour, where change is obviously an effect. Of course, to talk of a change in the basic facts is to use an extremely general description. But all descriptions are general to some degree. Even reference to movement of the wagon leaves much unspecified (for example, the speed of the movement). Using highly general terms seems strange only because, in practical life, we prefer the most specific description available. Here the aim is philosophical enquiry and that justifies the choice of a more general viewpoint.

Even in everyday life we show awareness of this necessary connection between contact and change. Everyone knows intuitively that two objects cannot occupy the same place at the same time. So we implicitly grasp that, when a moving body contacts a still one, change is necessary. To that extent, at least, our commonsense belief is reasonable. But our thoughts on such matters are never spelt out clearly. We may easily get carried away. Carelessly, we perhaps assume that contact must be followed by the specific event of movement, rather than the general event of change. Note, however, that reason is still being used here, though with less than perfect efficiency. The confusion is not due to the curious process of imagination described by Hume which is completely unrelated to reason.

Only when cause involves contact can we have any true sense of necessity. That fact underlies another feature of everyday thinking about causation. We are perplexed by causes where no contact seems evident. When we observe objects apparently attracting each other at a distance, we yearn for an explanation.

For example, we wonder how crumbs from the carpet are drawn into the vacuum cleaner. There happens in this case to be a satisfying answer. When the vacuum is created inside the cylinder, internal air pressure falls below that outside. External air then forces its way in, carrying with it small objects in its path. So the crumbs are not really attracted into the machine – they are pushed inside, by air.

Attraction is here explained away as displacement. If no such account is readily available, the causal relationship remains mysterious and magical. So, one object repelling another has a primitive appeal. But the thought of an object attracting another is repugnant. Repulsion is attractive and attraction repulsive. Because of this, the sights we most naturally associate with cause are those of collision and crashing, while the typical sound is the crack or crunch.

The defective commonsense notion of necessary connection was, for Hume, the whole story. In fact, behind everyday confusion lies an idea of respectable parentage. This is no misbegotten offspring of imagination, but the legitimate child of reason.

Trevor Emmott advises that the driving manoeuvres described in this article should not be attempted (at least, not in Sussex, where he lives).

Quotations are from Section VII of David Hume’s Enquiry Concerning Human Understanding (1777)