BACKGROUND ASSUMPTIONS

If we are going to subject a causal hypothesis to rigorous testing with the SCT and the NCT, we have to seek out a wide range of cases that might refute that hypothesis. In general, the wider the range of possible refuters the better. Still, some limit must be put on this activity or else testing will get hopelessly bogged down. If we are testing a drug to see whether it will cure a disease, we should try it on a variety of people of various ages, medical histories, body types, and so on, but we will not check to see whether it works on people named Edmund or check to see whether it works on people who drive Volvos. Such factors, we want to say, are plainly irrelevant. But what makes them irrelevant? How do we distinguish relevant from irrelevant considerations?

The answer to this question is that our reasoning about causes occurs within a framework of beliefs that we take to be established as true. This framework contains a great deal of what is called common knowledge—knowledge we expect almost every sane adult to possess. We all know, for example, that human beings cannot breathe underwater, cannot walk through walls, cannot be in two places at once, and so on. The stock of these commonplace beliefs is almost endless. Because they are commonplace beliefs, they tend not to be mentioned; yet they play an important role in distinguishing relevant factors from irrelevant ones.

Specialized knowledge also contains its own principles that are largely taken for granted by experts. Doctors, for example, know a great deal about the detailed structure of the human body, and this background knowledge constantly guides their thought in dealing with specific illnesses. Even if someone claimed to discover that blood does not circulate, no doctor would take the time to refute that claim.

It might seem close-minded to refuse to consider a possibility that some- one else suggests. However, giving up our basic beliefs can be very costly. A doctor who took seriously the suggestion that blood does not circulate, for example, would have to abandon our whole way of viewing humans and other animals, along with the rest of biology and science. It is not clear how this doctor could go on practicing medicine. Moreover, there is usually no practical alternative in real life. When faced with time pressure and limited information, we have no way to judge new ideas without taking some back- ground assumptions for granted.