Irrelevant material

Often arguments are embedded in other material which is not strictly speaking part of the argument. Sometimes this is an oversight on the part of the arguer – they haven’t noticed that some of their claims are irrelevant to their conclusion. But sometimes it’s not – perhaps the arguer has put the extra material in to provide the context for the the argument, or to catch the attention of the reader, or to fulfil any of the many possible purposes the arguer might have which are different from the purpose (which they must also have, since we’ve said they are an arguer) of providing reasons in support  of some conclusion.

One thing that you may have to do in reconstructing an argument is get rid of extraneous or irrelevant material – claims which are not premises in the argument because they are not relevant to the conclusion. Now we are going to talk a bit about what it means to say that a claim is irrelevant to a conclusion.

Suppose a student comes to my office and makes this argument (this, with minor variations, happens quite often):

​I deserve an A for this course.  If I don’t have an A average, I won’t get into second-year law, and if I don’t get an A for this course I won’t have an A average. 

Their conclusion is that they deserve an A for the course. But if you think about it, none of the rest of what they say backs up the claim that they deserve an A.  The kind of information that would back up the claim that they deserve an A would be to do with the quality of their work. For example, if they bring me their assignments and show me that actually their answers are much better than the grader has realised, so they should get As rather than Bs for their assignments, that would be relevant to the conclusion that they deserve an A for the course. But what they actually say has no bearing on the question of whether they deserve an A. Rather, they are giving reasons why they need an A. But “I need an A for this course” is a quite different conclusion.  (The student may be being sneaky. That you need an A for the course is not a reason for me to give you one – grades are determined by quality of work, not by need. So overtly trying to get me to give you an A by telling me how much you need one is unlikely to work.  On the other hand, if you can get your lecturer to think that you deserve an A, they might give you one. The problem is that if the lecturer has any critical thinking skills, they will be able to see that you’re not actually giving any reason to think that you deserve an A.)

What it means to say that a statement is irrelevant to a particular conclusion is that whether that statement  is true or false makes no difference whatsoever to whether we should believe the conclusion; it has no bearing on the conclusion at all.

When you are deciding about relevance and irrelevance, you first have to identify the conclusion. A statement is not relevant or irrelevant on its own – it’s only relevant or irrelevant to some particular conclusion.

Note that being irrelevant to a conclusion is not the same thing as being false. You can have irrelevant statements that are nevertheless true – for example, in our example, everything the student says about second-year law, etc, might be true, but that wouldn’t stop it from being irrelevant. And you can have false premises that are nevertheless relevant to the conclusion.

Here is an example. My colleague Clare works part-time, and she has a sign on her office door that says: “Clare is not here in the mornings.”  For the purposes of these questions, you should assume this bit of background information is true: My colleague Clare works part-time, and she is at work every afternoon but never in the mornings.

​Now it is 3 pm, and the sign says “Clare is out of her office having afternoon tea”, but in fact Clare has returned from her break and is in her office with the door closed marking essays. I want to know whether or not she is in her office.

The Clare example shows (we hope) the difference between relevance and truth.

Now that we’ve talked about what it means for a claim to be irrelevant  to a conclusion, we will look at an example of a passage of text in which some of the claims are relevant to the conclusion and some aren’t. Part of the job of reconstructing such an argument is to identify which claims are relevant, and put them into a standard form version of the argument as premises, while leaving out the claims that are irrelevant to the conclusion.

Here’s the example. My grandmother used to say to me, when I didn’t eat all the food on my plate: “Eat your dinner!  You’re being wasteful. There are children starving in Africa!” I was always puzzled about what the starving children in Africa had to do with me eating or not eating my dinner – after all, it’s not as though the food I was wasting was food that could otherwise have been eaten by them.  I think I was right to be sceptical. The argument really goes something like this (P2 isn’t stated, but I think it’s implied):

P1. Not eating all the food on your plate is wasteful.
P2. We shouldn’t be wasteful.
                                            
C. You should eat all the food on your plate.

I have not included “There are children starving in Africa” as a premise, because I don’t think it’s relevant to the conclusion – I have cleaned up the argument by leaving it out.