To justify adequately an assertion that one has made is in some sense the heart of argumentive reasoning. An inability to answer the “How do you know?” question suggests that the assertion should not have been made. In subsequent chapters, a case will be made that there are other aspects of argumentive reasoning that warrant close attention as well, but first it is essential to take a very close look at the arguments people offer to support the theories they espouse.

After subjects present their causal theories, the second segment of the interview for each of the three topics begins with the question, “How do you know that this is the cause —?” A number of probes follow, encouraging the subject to expand and become more specific, for example, “If you were trying to convince someone else that your view [that this is the cause] is right, what evidence [verbal emphasis] would you give to try to show this?” (See appendix 1 for full sequence of probes.) Probing ends with the question, “Is there anything someone could say or do to prove [verbal emphasis] that this is what causes —?” (If the answer is yes, the subject is asked to describe what it would be; if the answer is no, the subject is asked to explain why not.)

Subjects' responses to the request for evidence cover a very wide range. Not all or even a majority of responses, it turns out, fall into the category that we shall define as genuine evidence.