Once we have the claim or the point of an argument fixed in mind and have isolated the evidence or reasons offered in its support, the next question to ask is this:

Exactly how do the reasons offered in support of the conclusion work? In other words, what kind of guarantee — warrant — is provided to demonstrate that the reasons proffered actually do support the claim or lead to the conclusion? (A warrant in this context is like the warranty you get when you buy something.)

In ordinary and straightforward deductive arguments, warrants take different forms. In the simplest cases, we can point to the way in which the meanings of the key terms are really equivalent. Thus, if John is taller than Bill, then Bill must be shorter than John. We know this because we know what “is shorter than” and “is taller than” mean. If A is taller than B, it must be the case that B is shorter than A; those are the meanings of the phrases being used here. Of course, everyone involved does have to know the language well enough to understand the relationship between “is taller than” and “is shorter than.” The warrant in this case is the common understanding of what those two phrases mean.

In other cases, we may need to be more resourceful. A reliable tactic is to think up a simple parallel argument, an argument exactly parallel in form and structure to the argument we are trying to defend. If the two arguments really do have the same form and structure, and we are ready to accept the simpler one, then we can point out that the more complex argument must be accepted — because the two arguments have exactly the same structure. For example, if we want to argue that it is reasonable for FedEx to charge more for its delivery services than the U.S. Postal Service (USPS) does, we could point out that since it seems entirely reasonable to pay higher costs for special services from the USPS such as overnight delivery promised by a certain time (depending on factors such as location), then it is reasonable to pay even higher fees for similar overnight delivery by FedEx because that service includes sending someone to pick up what you want delivered.

In simple inductive arguments, we are likely to point to the way in which observations or sets of data constitute a representative sample of a whole population, even if not every member of the sample is strictly in evidence. For instance, when scattered information is plotted on a graph, the trend line does not have to touch each (or even any) of the data points as long as they are scattered above and below the line in roughly equal numbers in pairs that are roughly equidistant from the trend line. We can defend this projection on the grounds that it takes all of the points into account in the least complicated way. In such a case, the warrant is this combination of inclusiveness and simplicity.

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Establishing the warrants for our reasoning — that is, explaining why our grounds really do support our claims — can quickly become a highly technical and exacting procedure that goes far beyond the aims of this book. Even so, developing a “feel” for why reasons or grounds are or are not relevant to what they are alleged to support is important. “That’s just my view” is not a convincing warrant for any argument. Even without formal training, however, one can sense that something is wrong with many bad arguments. Here is one example: British professor C. E. M. Joad found himself standing on a station platform, annoyed because he had just missed his train. Then another train, making an unscheduled stop, pulled up to the platform in front of him. Joad decided to jump aboard, only to hear the conductor say, “I’m afraid you’ll have to get off, Sir. This train doesn’t stop here.” “In that case,” replied the professor, “don’t worry. I’m not on it.”