Introduction to Causal Arguments

One of the most important uses for inductive reasoning is to argue causation. Consider the following example:

A bicyclist moves into the traffic lane in order to pass a truck illegally parked in the bike lane. The driver of a car approaching from the rear slams on her brakes in order to avoid hitting the bicycle. A following car fails to stop in time, and smashes into the back of the first. The insurance companies disagree about who should be held responsible, and they go to court to decide who caused the accident.

What arguments are likely to be made in court? The bicyclist's lawyer will probably claim that the illegally parked truck caused her client to swerve into the lane of traffic. The lawyer for the driver of the first car will probably claim that the bicyclist's actions caused her client to slam on the brakes. The lawyer for the second driver will probably claim that the first car's sudden stop caused his client to smash into its back.

None of these claims seems to fit the pattern of an inductive argument, because none of them seems based on observation or experience. But, in fact, they do fit that pattern. The bicyclist's lawyer, for example, is actually arguing that:

• Normally the bicyclist would have continued in the bike lane, but in this instance he swerved into the lane of traffic.
• The only signficant difference between "normally" and "in this case" is the presence of the illegally parked truck.
• Therefore, the truck caused the bicyclist to swerve.

The lawyers for the drivers are making similar arguments: the first, that the only signficant difference was the swerving bicycle; and the second, that the only signficant difference was the suddenly braking car. Like inductive reasoning, then, these causal arguments are based on observed instances. (In this case, no observations are needed to convince us that the bicyclist would not normally have swerved or the first driver would not normally have braked suddenly. But if, for some reason, observations were necessary, we could design a study of automobile and bicycle traffic on that street, or survey drivers and bicyclists about their experiences, or in other ways provide evidence to verify the part of the premise describing the normal pattern of traffic. For more on this, see the section on Studies, Surveys, Polls, and Experiments.

These causal arguments, then, follow the form of an inductive argument with one important exception: whereas an inductive argument carries as part of its second premise the implication that there is otherwise no significant difference, these causal arguments carry the implication that there is only one significant difference: for the bicyclist, the truck; for the first driver, the bicycle; for the second driver, the first car.

How can we know that there is really only one significant difference? In real-life situations, we cannot usually be certain of that, since the world in which we live is a very complicated and intricate place. If, however, there is a strong likelihood of causation and there are no other apparent causes in evidence, then the argument will seem convincing. Two rules to remember in dealing with causation are:

1. The cause must precede the event in time. On one hand, arguments that have the effect before the cause are examples of the relatively rare fallacy of reverse causation. One the other, arguments whose only proof of causation is that the effect followed the cause are examples of fallacious post hoc reasoning.
2. Even a strong correlation is insufficient to prove causation. Other possible explanations for such a strong correlation include coincidence, reversed causation, and missing something that is the cause of both the original "cause" and and its purported "effect."

But what if, as the trial progresses, evidence is introduced to show that the bicyclist has a history of causing accidents by swerving in front of cars, or that the first driver has been involved in several accidents in which she caused her brakes to lock, or that the second driver was speeding and the street was slick? Such information could lead to very different causal arguments, where the implied claim does not concern "the only signficant difference," but rather "the only significant commonality." The bicyclist's lawyer might argue, for example, that her client's swerving may have caused the first driver to brake suddenly, but that the accident was caused by the way in which the first driver hit the brakes, and that she has caused similar accidents that way in the past. The first driver's lawyer, on the other hand, might argue that the accident was caused not by her client's braking, but by the second driver's unsafe speed, and the slick condition of the road, and that these factors have often caused other accidents. Both lawyers would be using a causal argument based on "the only signficant commonality."

Often, complex causal arguments use a mixture of "difference" and "commonality" reasoning, to show that the proposed cause is the only significant difference in cases where the effect did not occur, and the only significant commonality in cases where the effect did occur.

The strength of a causal argument, then, relies on three factors:

1. how acceptable or demonstrable the implied comparison is (for example, do we think that there is a basic similarlity in most respects between the circumstances of this accident and those of the many other times bicycles and cars have traveled on this street safely;
2. how likely the case for causation seems to be (for example, do we think that a bicycle swerving into an car's lane can cause an accident?);
3. how credible the "only significant difference" or "only significant commonality" claim is (for example, do we believe that the illegally parked truck is the only significant difference between this case and the many other times bicycles and cars went down that street without an accident?).