When we carry out an informal analysis of an argument, we pay close at- tention to the key words used to present the argument and then ask our- selves whether these key terms have been used properly. So far, we have no exact techniques for answering the question of whether a word is used correctly. We rely, instead, on linguistic instincts that, on the whole, are fairly good.
In a great many cases, people can tell whether an argument marker, such as “therefore,” is used correctly in indicating that one claim follows from an- other. However, if we go on to ask the average intelligent person why one claim follows from the other, he or she will probably have little to say except, perhaps, that it is just obvious. In short, it is often easy to see that one claim follows from another, but to explain why can be difficult. The purpose of this chapter is to provide such an explanation for some arguments.