By Daniel Tippens
Informal fallacies have become a hot topic in some circles. An informal fallacy is an argumentative move that may be psychologically persuasive, but is logically incorrect.  For example, an argument from authority is an informal fallacy, because it involves basing one’s conclusion on the premise that one is an expert with regard to something related to the conclusion. This can be quite persuasive. Indeed, many university classes only work because students buy into an implicit appeal to the expertise of the professor. That one is an authority on the matter, however, provides no guarantee of the truth of what one has said, and this is why such arguments are generally taken to be fallacious.
It is quite common for the charge of an informal fallacy to be taken as a decisive responses to certain arguments. It is considered a hands-down, “gotcha!” move. Recently, however, researchers have come to think that the kind of reasoning involved in informal fallacies may actually be sufficient justification for belief, or at least that informal fallacies may boost the justification for a belief. Let’s consider a well-known argument in the philosophy of mind known as the argument from hallucination. The argument goes like this:
Suppose that John is hallucinating a black whale.
- John experiences blackness.
- There is nothing that is black in front of John – the experience is a hallucination.
- A hallucination is a mental representation of something – a sense datum.
- Hallucinations and veridical (accurate) experiences are subjectively indistinguishable.
- Since the two experiences seem indistinguishable, they are identical.
- John’s veridical experience of blackness must also be a mental representation – a sense datum (from 5).
- All experiences are subjectively indistinguishable from their hallucination counterparts.
- All we are aware of are sense data.
This famous argument is essentially an argument from ignorance: a lack of evidence for -P is taken as evidence for P.
Let’s look at the move from premise 4 to premise 5. Since a hallucination of blackness is indistinguishable from a veridical experience of blackness, the two experiences are indistinguishable. In other words, since we have no evidence (introspectively) that a hallucination is not identical to a veridical experience, the two experiences are identical. A lack of evidence for non-identity is evidence for identity. So, despite the appeal to ignorance, the argument seemed to offer sufficient justification for the belief that we only are aware of sense data in our experience. What’s going on?
On the Bayesian account of informal fallacies, depending on the background conditions in which an appeal to an informal fallacy is made, the justificatory status of a belief that rests on the informal fallacy may vary. If I am talking with a known professional physicist, I’m justified in believing what he tells me about his field of study because he is an authority on the matter. But, if a biochemist tells me which car I ought to buy, on the grounds that he has a PhD in biochemistry, his appeal to authority does not justify the belief that I ought to buy this car, rather than that one. In the first case, the fact that a physicist is telling me about physics increases the probability that what he is telling me about physics is true, but in the second case, the fact that a biochemist is telling me about cars doesn’t increase the probability that his choice of car is the right one.
It is important to note that thus far, my discussion about informal fallacies has been restricted to how they relate to the justificatory status of a belief. But I also will want to consider whether an appeal to an informal fallacy, under conducive background conditions, warrants not only belief, but also acceptance, by which I mean that we treat whatever is under consideration as true for practical purposes, even though we don’t really believe it. I will conclude that there are occasions on which informal fallacies may confer justification for belief, but not acceptance. That is, they may justify my belief in something, but will not justify my acting as though the belief is true.
There are two reasons for pointing this out. First, many of the examples employed in the Bayesian approach to informal fallacies have practical undertones to them, so, it is worth demonstrating that acceptance and belief can come apart. Second, a significant part of the interest in informal fallacies comes from their practical consequences. Since my friend is an expert on wine, for example, I can trust what she says, when deciding which wine to purchase for dinner. But, if informal fallacies may fail to confer justification for acceptance, even while providing justification for belief, then we will need to be more careful when thinking about how an informal fallacy informs our decision-making process.
Acceptance and belief
Here, I take a belief in a proposition to have 3 features:
- A belief is a mental state in which somebody takes the world to be a certain way
- A belief is persistent.
- A belief disposes us to report that we take the world to be a certain way.
What it means for a person to take the world to be a certain way is just to say that he endorses a certain proposition as accurately reflecting a certain state of affairs. If I believe the world is round, for instance, then I take the world to be round. A belief is persistent, insofar as I continue to hold it and so long as I don’t encounter any defeating evidence. Of course, when I am asked if I think the world is such-and-such, I will answer, “yes,” if I believe the proposition. Belief disposes us to report how we take the world to be.
Turning, now, to acceptance, it has the following features:
- Acceptance is a mental state in which somebody treats a proposition as true for practical purposes.
- Acceptance may or may not be persistent.
- We may be hesitant to report that the world is a certain way, when asked.
To treat a proposition as true for practical purposes is just to assume the proposition to be true, while not necessarily believing it. For example, a physicist might believe that quantum mechanics is true, but not accept it. If he wants to figure out how to get a satellite into orbit, he may treat Newtonian Mechanics as true for practical purposes. 
Acceptance may or may not be persistent. Once a physicist has figured out how to send a satellite into orbit using Newtonian mechanics, he may never accept Newtonian mechanics again. However, sometimes acceptance is persistent. A person might not believe there is an external world, but he may accept that there is an external world for his entire life. David Hume and Thomas Reid are good examples of philosophers who accepted that there was an external world, even though they didn’t believe it.
Belief and acceptance can overlap. I both believe and accept that my houseplant needs to be watered, in order to survive. But as indicated above, they can also come apart and so can their epistemic statuses. As already seen, I might be justified in accepting a proposition, but not justified in believing it. But there are also cases where I might be justified in believing a proposition, but not accepting it. This kind of case is what we will investigate in the next section.
It would be difficult to classify which kinds of factors uniquely justify acceptance over belief and vice versa. I cannot do so here. But what is important, as mentioned before, is that belief and acceptance come apart. I can believe something but not accept it for pragmatic reasons. Let’s now take a look at how this happens in the context of informal fallacies.
Acceptance, belief, and informal fallacies
Let’s examine a case in which both a belief in and acceptance of some proposition is justified by appeal to an informal fallacy. 
- There is no evidence to suggest that drug x is not safe, so I have reason to believe that drug x is safe.
(1) seems to confer justification for the belief that drug x is safe, when certain background conditions are met: past clinical trials have shown no detrimental effects or increased mortality rates, etc. When past studies have given us no reason to believe that a drug is unsafe, it seems reasonable to believe that the drug is safe, since the lack of evidence in those past studies increases the likelihood of the drug’s safety. Additionally, should we fall ill, most of us would use the drug, once we have learned the facts just mentioned.
Now consider the following two scenarios. These are adapted from a paper by Jacob Ross and Mark Schroeder :
Sam is well-known competitor in memory events. He is a professional when it comes to remembering things: people, events, whatever. Additionally, he remembers things in great detail. However, you don’t personally know Sam very well. You have been introduced to him just today when a mutual friend asked you if Sam could crash on your couch. But your friend did tell you about how Sam is a well-known professional “rememberer.”
You are preparing your lunch for the next day with Sam. You make your favorite sandwich, a classic peanut-butter and jelly sandwich. Sam makes his standard lunch: ham and cheese. You both put your sandwiches in plastic bags and place them into the refrigerator. Sam places his on the right-hand side, and you place yours on the left-hand side. In the morning, you both reconvene in the kitchen to grab your sandwiches. However, you have forgotten if you put your sandwich on the left or on the right. Sam says, “you put your sandwich on the left. Trust me, I am a rememberer.” So, you grab the sandwich on the left and eat it. Sam was right.
In this case, you have appealed to the informal fallacy of appeal to authority. Sam is an expert rememberer, so when he tells you that your sandwich is on the left, you seem justified in believing him. Additionally, you seem justified in accepting the proposition that your sandwich is on the left. Indeed, you ate it. But now take a look at case two.
All the conditions from case one are the same, except that after you both reconvene in the kitchen to grab your sandwiches, you see a sign on the refrigerator that says, “The ham and cheese sandwich has been poisoned, and if you eat it you will have a bad headache for 2 days. The placement of the sandwiches has not been altered. Cheers.” Sam says, “don’t worry, I remember that you put your sandwich on the left.” You don’t eat the sandwich. 
In this case, you still seem justified in believing Sam. What you don’t seem to be justified in doing, however, is accepting what he says. Indeed, you are likely to respond to Sam, “I believe you, man. I really do. But I just can’t risk eating the sandwich on my left.” The risk associated with him being wrong is just too high for you to accept what he says, in your decision-making.
In the first case, an appeal to authority justified your belief that Sam was right, as well as your acceptance of what he has said. In the second case, though your belief was justified, your acceptance was not. This kind of situation happens often, when high-risk practical problems undermine justification for acceptance, but not belief.
The situation just described is one in which there is a substantially high risk in taking the wrong advice. But, many cases in which we appeal to informal fallacies involve more difficult tradeoffs between potential risk and benefit. Indeed, I think this sort of problem arises whenever we appeal to informal fallacies for practical purposes, and thus, we must be careful not to conflate justification of belief with justification of acceptance
A significant reason why we are concerned with the justificatory benefits of informal fallacies is because of the role they may play in practical decision-making. Informal fallacies may justify beliefs, when taken against certain background conditions, but they may not always justify acceptance of those same beliefs, under those same conditions. It is worth remembering, then, that just because an informal fallacy confers justification for a belief in a proposition, it may not also confer justification for that proposition’s acceptance.
Daniel Tippens is co-founder of The Electric Agora. He is also a research technician in the S. Arthur Localio Laboratory at New York University School of medicine.
- Hahn, Ulrike; Oaksford, Mike; The Rationality of Informal Argumentation: A Bayesian approach to reasoning fallacies, Psychological Review, 2007.
- Much of this section comes from Ross, Jacob; Acceptance and practical reason, Dissertation (Rutgers), 2006.
- See endnote .
- Ross, Jacob, Schroeder, Mark; Belief, Credence, and Practical Encroachment, Philosophy and Phenomenological research, 2011.
- It is worth noting that Ross and Schroeder use cases like this in the context of showing how pragmatic factors can affect one’s credence, I instead used these cases to show something about acceptance.