Mapping the Relations Between Arguments
Our thinking isn’t usually made up of single, isolated arguments, but of complex sets of interrelated arguments. And argument mapping helps us to visualize and understand these relations. In this section we’ll consider how the argument we looked at earlier concerning Natalie might be related to other arguments. Before we do that, let’s review the simple map of that argument and get clear on how we are going to refer to each of its parts.
| Map 1 |
Each box contains a numbered proposition. We’ll refer to them as “Proposition 1,” “Proposition 2,” and “Proposition 3.” The green circle represents the act of inference, and we’ll refer to it as “Inference A.” We’ll use the phrase “Argument A” to refer to the whole argument, which includes the inference, along with its premises and the conclusion.
With this terminology under our belts we can look at and discuss more complex maps that show relations between arguments.
One way in which arguments can be related is by sharing the same conclusion. Here’s an example:
| Map 2: |
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Argument B has the same conclusion as Argument A. They are two distinct arguments because they give separate reasons for believing the conclusion. To see that the reasons are separate, pick one of the premises from either argument, and think about what other premises you would need to combine it with before it would give you a reason to think that Natalie murdered Carl. You will see that in order for Proposition 1 to convince you that Natalie murdered Carl, you would need to also know Proposition 2, but that you wouldn’t need to know Propositions 4, 5, or 6. Likewise, in order for Proposition 5 to convince you, you’d need to also know Propositions 4 and 6, but not Propositions 1 or 2.
Chains of Argument
A second way arguments can be related is that a premise of one argument can be the conclusion of another, forming a chain. Here’s an example of that:
| Map 3: |
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Notice that Argument A (from Map 1) is part of this larger map. One of its premises is now the conclusion of another argument, labeled C, and one of C's premises is a premise for a further argument, labeled D. So, we can think of D, C, and A as a chain of arguments, with Proposition 3 as their ultimate conclusion. (Or, to put it another way, we can think of D, C, and A as making up a three-step argument for Proposition 3. And, we can think of Propositions 2 and 8 as intermediate conclusions of this larger argument.)
Complex Maps
Now let’s consider a larger map that includes all the arguments we’ve looked at plus three others.
| Map 4: |
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Argument E is a third argument for Proposition 3. I expect you’ll agree that it a bad argument. The map shows two objections to this argument, each symbolized by a red octagon. Objections are (in effect) arguments that something is wrong with the argument they're objecting too.
Objection H objects to Proposition 11, which is a premise of Argument E. When someone objects to a premise, he is arguing that it is not a suitable premise. This could be because the premise us false (as is the case with Proposition 11), but there are other reasons why a premise might not be suitable. For example, we might not be in a position to tell whether it's true, which would make it useless as a premise. Or we might only have reason to think that it's true if we already believe the conclusion, in which case it would be a case of circular reasoning. In addition to objections to premises, there are objections to inferences. These are arguments that the inference is faulty.
Objection I objects to Inference E. To object to an inference is to argue that it's not a good inference—that, even we knew the premises to be true, the inference wouldn't give us a reason to believe the conclusion. We'll discuss the factors that make for a good or bad inferences later.
Inference F is represented by a red inference symbol instead of the usual green one because it represents a counterargument. Counterarguments are an arguments against conclusions, rather than for them. Counterargument F gives us a reason to think that its conclusion, Proposition 3 is false. If we wanted to treat F as a regular argument (represented by a green symbol), then it's conclusion would be that “Natalie did not murder Carl.” We could write this conclusion out as a distinct proposition, in its own box, but since it is equivalent to saying that Proposition 3 is false, it’s nice to have a way to relate F to Proposition 3 on the map, so that we could see a glance that F is arguing against the very same conclusion that Arguments A, B, and E are arguing for. This is the purpose of the red counterargument symbol.
Argument G supports Proposition 14, which is a premise for Counterargument F.
The point of an argument is to help us tell what’s true. So, if we end up in a situation where we have arguments with opposite conclusions as we do in Map 4, then there must be an error somewhere. In this map, Objections H and I, indicate what's wrong with Argument E. But even if we eliminate Argument E, we have two other arguments for Proposition 3, and we have a counterargument against this same proposition. Proposition 3 cannot be both true and false, so something must be wrong either with Arguments A and B or with Counterargument F. Let’s turn now to the things that can go wrong with arguments, and why some arguments are better than others.