Lesson 10: the Rules of Simplicity and Consistency

Chapter 5

Lesson 10: The Rules of Simplicity and Consistency

The following is an event that occurred in Arizona on the night of March 13th, 1997:

Hundreds of people over a distance of several hundred miles looked up at the night sky after 8:00 pm and saw a set of five to seven lights in a V-formation moving slowly and noiselessly from north to south. Almost two hours later, nine balls of bright light appeared in the sky south of the city of Phoenix and seemed to hover there for several minutes until they slowly disappeared.

At least two theories have been proposed to explain these events. (1) A squadron of military fighter planes, many miles high, passed over the state in a V-formation. The balls of light were flares dropped at high altitude by military planes. (2) A V-shaped alien spacecraft, probably from a planet outside of our solar system, passed over the state. Later that same night, alien spacecraft hovered near Phoenix before flying away.

Videotapes taken of the lights seemed to verify many of the eyewitness reports: there actually were strange-looking points of light in the sky that night. Given the evidence presented, which theory would you choose as more likely to be true? You learned in the preceding lesson that theories are tested by deriving predictions and making observations to see if the predictions are confirmed. If the lights actually were alien spacecraft, we might predict that officials would deny that military aircraft were in the area that night. In fact, military officials did deny (initially, at least) that military aircraft flew over the Phoenix metropolitan area on March 13th, 1997. Of course, this prediction also is consistent with the other theory: military officials might deny the existence of the flights because they wanted to keep their activities secret. As this example suggests, it can be very difficult to derive testable predictions that allow us to determine which theory is more likely to be correct.

But there are other criteria we can use to determine which theory is a more likely to be true. In order for the alien-spacecraft theory to be true, we would have to assume that aliens are visiting the earth and that they are able to travel at least close to the speed of light (since their home planet is likely to be at least several light years away). In order for the military-aircraft theory to be true, we would only have to assume that military aircraft flew over the state of Arizona that night. Because there is no reliable evidence that alien spacecraft are visiting earth, and because what we know about the universe tells us that it would be nearly impossible for an object with the mass of a spacecraft to travel anywhere near the speed of light (but see The Speed-of-Light-Limit Argument, undated), it seems more reasonable to believe that the lights were associated with military activities. The general point being made here is this: the theory that makes fewer untested claims, and contains claims in agreement with other things we know, is more likely to be true. In other words, in deciding whether or not to accept a theory, we need to consider which theory is both simpler and more consistent with what is known about the universe. These two criteria may be called the "rule of simplicity" and the "rule of consistency." Both rules are "rules of thumb" (procedures based on experience or practice): they help us when we are evaluating different theories but they do not always lead to a correct solution.

The Rule of Simplicity

In everyday life, it is never a good idea to make too many assumptions when trying to explain something (an assumption is an untested claim about an event). For example, when a person is late for an appointment, we often make the assumption that the person is still alive; but it may turn out that this assumption is wrong: the person might be late because he or she has died. This example suggests that, as the number of assumptions becomes larger, the likelihood of an incorrect conclusion becomes greater. Because of this, scientists generally try to make the fewest assumptions necessary when attempting to develop a theory about a phenomenon. This is referred to as the rule of simplicity, which states that, when two theories make the same predictions and explain the evidence equally well, the theory that makes the fewest assumptions is more likely to be true. The rule of simplicity was summed up well by Albert Einstein, who said: "Everything should be made as simple as possible, but not simpler" (quoted in Gibbs & Hiroshi, 1997).

The rule of simplicity (also called the principle of parsimony) was often mentioned in the philosophical writings of a fourteenth-century English monk named William of Ockham (thus, the rule sometimes is called, "Occam's Razor"). According to William of Ockham, the rule states that "plurality should not be posited without necessity" (Carroll, 2002). This means that a theory should assume only the minimum number of factors needed to explain whatever it is we are trying to explain. For example, if we are trying to explain why a person looks like she is asleep in class, it is best to start with the explanation that she fell asleep because she did not get enough sleep last night and, therefore, was very tired. On the other hand, we might speculate that the student was only pretending to be asleep because she was angry with the instructor and was trying to make him feel bad: she hoped that he would think that she was asleep because he is a boring teacher. Although this explanation is possible, it "posits plurality without necessity,", which means that it includes more assumptions (she is pretending, she is angry, she is motivated to make the instructor feel bad, instructors feel bad if they believe that they are boring, the instructor will believe that he is boring if he sees a sleeping student) than does the other explanation (she didn't get enough sleep, she was tired). If we gather more evidence, we might find that the more complicated explanation is more likely to be true (for example, she might tell us, "I was only pretending to be asleep because I was so angry with him!"). But, until such evidence is forthcoming, it is best to start with the simpler explanation.

The Rule of Consistency

Whenever we try to explain something, we always make various assumptions, many of them automatically (that is, without conscious thought). For example, when trying to explain why a person is late for an appointment, we usually assume automatically that time passes at the same rate in the person's location as it does in our location, that the person is still somewhere on the face of the earth, and that the person's body is visible (that is, that he or she has not become invisible). In other words, in everyday life, the assumptions we make about events generally are consistent with what we already know about how the world works.

The same is true in science. Scientific theories generally make assumptions that are consistent with what is already known about the subject matter being studied. This may be referred to as the rule of consistency, which states that, when two theories make the same predictions and explain the evidence equally well, the theory that is more consistent with what is already known is more likely to be true. In other words, the rule of consistency (also called the principle of conservatism) states that, if a particular claim is well established (there exists a great deal of evidence that supports the claim), then a new claim that conflicts with it is not likely to be true. For example, if someone claimed that he drove his car from New York City to London, you would probably conclude that the person was either lying or mentally disturbed. Why? It is well established that an ocean separates these two cities, cars don't float, and there are no gas stations in the ocean between New York and London. Thus, it seems very unlikely that he could have driven his car from one city to the other. On the other hand, you might eventually discover that his car is solar-powered and is shaped like a boat. In this case, you might change your mind about his claim because, now, it is consistent with other well established claims (boats can float and a solar-powered car would not need gasoline). But until such evidence is forthcoming, it is best to tentatively reject the claim.

One last thing that needs to be emphasized about this lesson is this: the rules of simplicity and consistency are only "rules of thumb." That is, they are only approximate rules and, therefore, should not be followed slavishly. If the evidence leads us to a more complex explanation or to an explanation that seems to violate what we think we know about something, then we need to follow the evidence. As has been stated in a number of lessons, the causes of virtually all phenomena we wish to explain in psychology are characterized by multifactorial causation (that is, complex causation); and sometimes we discover, by noting an inconsistency, that a theory is incorrect in an important way. In the end, both in science and in everyday problem-solving, the evidence should always trump any other consideration.

Critical Thinking Questions

Question 10-1 Which rule, simplicity or consistency, do you think would be of most help in deciding whether to reject each of the following claims? Briefly explain each answer.

 High self-esteem is associated with violent behavior  Ten-year-old Keri's high self-esteem is due to the combined influence of a number of factors: her parents are strongly involved in her life; her parents constantly demonstrate unconditional acceptance of her; she compares herself with a low-ability reference group; and she believes that others who evaluate her negatively do so because of their own problems.  Paul prefers to have as friends people who don't like him.