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Because How to Analyze and Evaluate Ordinary Reasoning Weak Reasons and Argument from Ignorance |
G. Randolph Mayes Department of Philosophy Sacramento State University |
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Weak reasons
We have already learned that weak rationales are sometimes the result of weak or unreliable principles. Weak rationales can also result from weak reasons. As opposed to principles, which we typically evaluate in terms of their reliability, we evaluate reasons in terms of truth and falsity, more specifically the likelihood or probability that they are true or false. In the simplest case, a weak reason is simply a reason that seems very unlikely to be true. For example:
The rationale would look like this.
The principle here is quite reliable, but suppose you know that Roger himself owns several cats. This would give you grounds for doubting that Roger is actually allergic to cats. Of course it is still possible that he is allergic to cats, and there may be a good explanation why Roger has cats even though he is allergic to them. (Perhaps Roger is not allergic to the kind of cat he owns or perhaps he keeps the cats outdoors.) Still, you have quite strong grounds for thinking the reason is unlikely to be true and it would be appropriate to call this a weak reason until further support has been given.
This is the definition of weak reason.
The proper analysis of the above example would be:
Note that we could not provide such an analysis if we didn't know anything about Roger, and this is typical of weak reason criticisms. In order to make the criticism effectively you need to provide some basis for doubting the reason. It's not enough to simply say that the reason has no support. (cf. Argument from Ignorance below.) Note also that the definition of weak reason is similar to our definition of weak principle in that it makes reference to the degree of confidence with which the reason is expressed. This is important because reasons that are not highly likely to be true can still be credible enough for a particular context or adequate to provide qualified support for it's conclusion. For example:
This example could be reconstructed as an argument or an explanation. When we reconstruct it as an explanation, the rationale looks like this.
Notice again that the principle is very reliable. Assuming that Trent actually remembers correctly where he last parked his car, you might point out that the reason isn't obviously true because there are other reasons that would explain a missing car. Perhaps one of his friends borrowed it, or perhaps it was repossessed or impounded. But unless you have an independent basis for thinking that these other causes are more plausible, you should not criticize Trent's reason as a weak reason in this case, especially since he has only presented it as a hypothesis to explain the fact that the car is missing. It's worth noticing here that if we had reconstructed Trent's reasoning as an argument you would have approached the criticism of this rationale differently:
Now you will notice that, while the reason is no longer dubious, the principle is not very reliable. Since Trent has only said that his car might have been stolen you might think this principle is strong enough for the degree of confidence Trent has expressed in it. On the other hand, if in claiming that it might have been stolen you understand Trent to be saying that this is the likeliest possibility, it would be reasonable to criticize the principle as weak, since other available principles are stronger (e.g., If object x is missing, then x was misplaced.)
Here is a another straightforward example of a rationale with a weak reason, and it's corresponding analysis.
Again, if you are a person who incorrectly believes that Vitamin C prevents the common cold, you would not be in a position to provide this analysis.
Weak reasons in reasoning chains
Weak reasons in argument chains
With arguments wewill typically focus a weak reason criticism on the terminal reason in a reasoning chain because that is the reason for which no further support is given. Weakness in a reason other than a terminal reason can almost always be traced to weakness in the reasons or principles that support it. For example:
This is obviously a very flawed rationale. The most straightforward criticism to make of it is that P1 is highly unreliable. You might also claim that R1 is a weak reason. After all, the only evidence given for it is Roger's own testimony, who may simply be a wayward child who resents his well-deserved punishment. Notice, however, that this really turns out to be a criticism of P2. You are saying that just because a person says something doesn't mean it's true, especially if the person is biased or unreliable. So the weak reason analysis doesn't really add anything to the weak principle analysis in this case.
Weak reasons in explanation chains
The weak reason criticism works differently for explanations, because the reasons in explanations are causes, not evidence. Hence, when we say that a reason in an explanation is weak, we are not saying that the rationale requires more causes.. Rather, we are saying that an argument needs to be given that one or more of the causes in question actually occurred. To see this, consider the following explanation.
Now suppose you have independent reasons for doubting R1. Perhaps you drove by their place last night and you're pretty sure you saw May and Jack making out on the porch swing as usual. This would be a basis for claiming that R1 is a weak reason. But note that in this case, to say that R1 is weak is not to say that R2 and P2 together provide insufficient evidence for R1, for they do not provide evidence for R1 at all. To say that R1 is weak is just to say that it requires evidence, and this is a criticism that can be legitimately brought to bear on any claim in an explanation.
Weakness stemming from reasons and principles
Weakness in a rationale can be due to a combination of weakness of reason and weakness of principle. In fact, sometimes a rationale can contain a fairly strong principle and a fairly strong reason, yet we will criticize the rationale as weak because, taken together, they do not justify the degree of confidence expressed in the conclusion.
The corresponding rationale might look something like this.
This is the sort of reasoning we have to evaluate on an everyday basis. You can easily imagine the ensuing conversation.
You will notice that in very short order Jess has exposed both that she does not have high confidence in the reason, and that the principle itself is far from reliable as well. When evaluating rationales it is important to understand that the confidence we attach to a conclusion is a multiplicative function of the confidence we attach to the reason and the conclusion. In other words, if Jess were about 80% confident in the reason and regarded the principle to be about 80% reliable, then on the basis of this reasoning she should only be about 64% confident (.8 x .8) in the conclusion. (If this seems mysterious to you, an analogy might help. Suppose you are waiting for someone at the train station and you are wondering whether she will show up on time. Suppose you think there is about an 80% chance that she caught the train. If the train is perfectly reliable, then that would mean that there is an 80% chance that she will show up on time. But if the train itself is only 80% reliable, then the likelihood that she will show up on time is less than 80%. With some important qualifications we'll ignore for now, the probability that she will end up at the station on time in this case is the product of probability that she will catch the train and the probability that the train will arrive on time or .8 x .8.= .64) In the case of Jess and the French guy it would be reasonable to give the following analysis.
Argument from Ignorance: Flawed weak reason and weak principle analyses
Finally, it is important to realize that reasons and principles may seem weak to us simply because we are uniformed about a subject. For example, within evolutionary biology it is universally acknowledged that chimpanzees and humans have a recent common ancestor. This explains why humans and chimps are more physically and genetically similar to each other than they are to other existing species. Now consider the following rationale:
Someone who has never studied evolutionary biology or who was raised to believe that humans and animals have completely different origins would understandably be inclined to believe that the reason in this rationale is weak. Of course, it is not weak at all, it is just that the reason lies outside this person's domain of knowledge.
The tendency for people to dismiss claims because they are unfamiliar with the evidence in favor of them is a logical fallacy called Argument from Ignorance, which may be understood as follows:
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