But, if you think about it, there is no point in including P2 in the reconstruction of the argument. PI alone is the basis for thinking that the restaurant will be fully booked; that the arguer once dined there with Professor Gilmour is irrelevant. When a proposition stated by the arguer is irrelevant to the reasoning which delivers the conclusion, that proposition should not be included in a reconstruction of the argument.
It might seem that the only reason not to include irrelevant material in an argument is that it would be distracting. But in fact it can affect our assessment of the argument in a more important way. For suppose PI is true, but P2 is not; the arguer, wishing to boast of a greater intimacy with Professor Gilmour than they in fact enjoy, fibbed when telling us that they dined out with him. If we were to discover this, yet persist in the above reconstruction of the argument, we should have to pronounce the argument to be unsound. Indeed, upon discovering the lie and becoming annoyed with the arguer, we might well be eager to do just this. But this would be wrong, because P2 is irrelevant to the argument. The argument should be represented like this:
P1) Les Champignons is usually fully booked.
C) (Probably) Les Champignons will be fully booked tonight.
Since PI is true, and the argument is inductively forceful, the argument is inductively sound. The arguer's snobbish mendacity is irrelevant.
This is an obvious point, but some cases are more subtle. In such cases it is a point that is easy to lose sight of, especially when we wish to refute the argument. Consider this example:
If the tuna industry is not regulated more stringently, it will collapse altogether, because tuna populations will vanish. The evidence is obvious: tuna catches have decreased significantly every year for the past nine years. Indeed, because of depleting stocks, the Mid-Pacific Tuna company went out of business.
Suppose we initially reconstruct the argument as follows:
P1) Tuna catches have decreased significantly every year for the past nine years. P2) Last year, the Mid-Pacific Tuna company went out of business.
C) If the tuna industry is not regulated more stringently, it will collapse altogether.
For the moment, we leave out connecting premises. Here the arguer is claiming that PI supports the conclusion by itself. But the arguer also seems to regard P2 as providing some additional evidence, in the form of an inductive inference from one case to the general proposition that tuna stocks have declined sufficiently to cause difficulty for the tuna industry as a whole. So surely P2 is not completely irrelevant to the argument. Both PI and P2 support the conclusion (it might be plausible to say that they support it jointly rather than independently - review the section on evidence in Chapter 3 -but here we can ignore this point). Suppose now you discover that although PI is true, P2 is false, because the Mid-Pacific Tuna company did not go out of business; rather, it was taken over by the larger Pan-Pacific Tuna company. You might then conclude that since P2 is false, the argument is unsound, end of story. You might, indeed, exploit this mistake in attempting to discredit the arguer. If you were a person who opposed greater regulation of tuna fishing, drawing attention to this factual error might be a good rhetorical strategy to use in persuading people not to listen to arguments for increased regulation. Nevertheless, it would be a mistake from the point of view of critical thinking. This is simply because PI, by itself, constitutes a good reason for accepting the conclusion. To fixate upon the falsity of P2 simply diverts attention from the arguer's having cited PI. According to the Principle of Charity, we should simply omit P2 in our final reconstruction of the argument. More exactly: both PI and P2, quite independently of each other, would support the conclusion. So really we should regard the arguer as having given two arguments for the conclusion, one of which we know to be unsound. So according to the Principle of Charity, we should now focus on the other argument, the one utilising PI. Adding the needed connecting premises and an intermediate conclusion, we get:2
P1) Tuna catches have been decreasing significantly for the past nine years.
P2) If tuna catches have been decreasing significantly for the past nine years, then the tuna population will vanish unless the tuna industry is regulated more stringently.
C1) The tuna population will vanish unless the tuna industry is regulated more stringently.
P3) If the tuna population vanishes, the tuna industry will collapse altogether.
C2) If the tuna industry is not regulated more stringently, it will collapse altogether.
2 It might be more plausible to reconstruct this argument as inductive rather than deductive, but we have ignored this in order to maintain the focus on the point about relevance.
This reconstruction makes for a plausible argument as it stands, but there are two further points about it that it may be useful to make.
You might find it hard to see that the argument from CI and P3 to C2 is valid. This is probably because of the word 'unless', which many people find confusing. But this problem is easy to get round. Remember from the discussion of conditionals that, in almost every sentence in which it appears, the word 'unless' can be removed, and replaced with 'if . . . not'. For example, the consequent of P2, and thus CI, can be rewritten as:
The tuna population will vanish, if the tuna industry is not regulated more stringently.
This sentence is itself equivalent to this:
If the tuna industry is not regulated more stringently, then the tuna population will vanish.
which you might find easier to work with. Now it is easier to see that the argument from CI and P3 to C2 is valid. Suppose that the tuna industry is not regulated more stringently. Then according to CI (as rewritten above), the tuna population will vanish. But if so, then according to P3, the tuna industry will collapse. So it follows from CI and P3 that if the tuna industry is not regulated more stringently, then the tuna industry will collapse; which is exactly what C2 says. (This is an example of a 'chain argument', as discussed near the end of Chapter 2.)
Returning now to the main theme of this section. The basic moral of this section is that the truth-values of the premises actually advanced by an arguer can be more or less relevant to the soundness of the argument. Sometimes it is highly relevant that a given premise is false, sometimes it is much less so. It depends upon the nature of the mistake, and upon the role played in the argument by the premise. The degree of relevance must therefore be taken into account in the process of reconstruction.
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