15.5.1: Probabilistic and Deterministic Explanations

The best explanations of an event usually give us a good reason to have expected the event. Suppose you want to explain why apples fall of the apple tree and hit the ground. One untestable explanation would be that it was the apple's "time" to leave the tree. That explanation appeals to a supernatural notion of fate or destiny. A scientific explanation is that the apple fell because it absorbed enough water through its stem that its weight increased above the maximum downward force that the brittle stem could resist.

Because explaining people's behavior is harder than explaining the behavior of apples, the current principles of psychology are less precise than the principles of physics. Psychologists depend on rules of thumb; physical scientists have deterministic laws that indicate what will happen rather than what might happen. For example, why did Sarah decide not to go out with Wayne when he mentioned he had an extra ticket to the concert? After talking with her, a psychologist might explain her action this way:

1. Wayne suggested that Sarah spend her time doing something she believed wouldn't be interesting to her.

2. People will not usually do what they have little interest in doing, nor what they perceive to be against their self-interest.

Sentence 1 states the relevant initial facts of the situation, and sentence 2 expresses the relevant law of psychology. This law is less precise than the law of gravity. It is only probabilistic, not deterministic, because it doesn't say what will happen but only what probably will happen. Using 1 and 2 in advance, we could predict only what Sarah probably would do, not what she will do. Psychology can't give a deterministic explanation. Such is the current state of that science.

Suppose you asked why you can see through glass but not through concrete, and you were told: "Because glass is transparent." That answer is appropriate for an elementary school student, but not for a more sophisticated audience. After all, transparent merely means being able to be seen through. The explanation is trivial. Up until 1926, however, no one had a better explanation. Glass's being transparent was just one of the brute facts of nature. It was accepted, but no deeper explanation could show why. Then, in 1926, the theory of quantum mechanics was discovered. From the principles of quantum mechanics, it was possible to deduce that anything made of glass should permit light to pass through. Similarly, quantum mechanics allowed us to find out why water is wet. These examples illustrate two main points: (1) General theories are more valuable than mere collections of specific facts, because with a general theory you can explain a large variety of individual facts. (2) If you can deduce a phenomenon from some well-accepted principles, you have a much deeper explanation of the phenomenon than if you can't carry out this deduction.