12.3 Suggestive Evidence

We have already studied the evidence we get directly, such as through observation, and the evidence we get through inductive inferences based on similar cases. A third category, the weakest sort of evidence, occurs when a hypothesis would explain the other evidence we have.

Table of Contents

• 12.3 Suggestive Evidence
  • 12.3.1 Abduction and Hypothesis
    • Abductive Argument
  • 12.3.2 Disanalogies and Exceptions
    • The Plausibility of a Hypothesis
    • Identifying Dissimilarities
  • 12.3.3 Inference to the Best Explanation
    • Inference to the Best Explanation
    • The Seven Explanatory Virtues
    • Falsifiability
  • 12.3.4 Moral Arguments
    • The Top Ten Types of Moral Argument
    • 1. Arguments by Analogy.
    • 2. Arguments from Function.
    • 3. Arguments from Reciprocal Relationship.
    • 4. Arguments from Authority.
    • 5. Arguments from Consistency.
    • 6. Arguments from Humanity.
    • 7. Arguments from Justice 
    • 8. Arguments from Consequences.
    • 9. Arguments from Freedom.
    • 10. Arguments from Responsibility.
    • Conclusion
  • Submodule 12.3 Quiz

12.3.1 Abduction and Hypothesis

One explanation for erratic driving is that the driver is under the influence.

Abductive Argument

An abductive argument is an invalid form of argument: in fact, it’s a form of argument which we earlier classified as a fallacy. Recall the the fallacy of affirming the consequent:

               1. If John did the crime, then John’s DNA is at the scene.

               2. John’s DNA is at the scene.

               C. John did the crime (!!INVALID!!)

It is easy to see why this argument is invalid, since here are many other ways in which John’s DNA could have come to be at the scene besides being the criminal. Nonetheless, there is something about the presence of John’s DNA that suggests that John did the crime. If John did the crime, then that would explain why John’s DNA is at the scene. Again, consider this invalid Categorical Syllogism, which is also an abductive argument:

               1.  All cats have tails and fur.

               2. Some of Jane’s pets have tails and fur.

C. Some of Jane’s pets are cats. (!!INVALID!!)

Again, this is invalid, since Jane might have only rabbits as pets, which also have tails and fur. Nonetheless, that she has pets with tails certainly suggests, or at least makes it not unreasonable to think, that she might have a cat, as opposed to a pet lizard or a pet jellyfish.

We might reconstruct these abductive arguments as valid deductive arguments, where the conclusion becomes one of the premises, acting as a hypothesis which would serve to explain something we know to be true:

               1. If John did the crime, then John’s DNA is at the scene.

               2. John did the crime. (Hypothesis)

               C. John’s DNA is at the scene. (Fact to be explained) (1, 2 MP)

Here is another example:

               1.  All cats have tails and fur.

               2. Some of Jane’s pets are cats. (Hypothesis)

C. Some of Jane’s pets have tails and fur. (Fact to be explained) (1, 2 CS)

Abductive reasoning, in other words, is a logical method for formulating a hypothesis. The word “hypothesis” makes many people think of a “guess”, but formulating a hypothesis isn’t a matter of guesswork. Rather, it’s a matter of thinking through what would explain the information we already know.

Here are two more examples of invalid arguments, which we are earlier identified as fallacies, but which can still suggest their conclusion as abductive arguments. First, there is denying the antecedent:

               1.  If Sam accepted the job, then Sam will buy a new car.

               2. Sam did not accept the job.

               C. Sam will not buy a new car. (!!INVALID!!)

Second, there is denying a disjunct:

               1. Either Sam accepted the job, or Sam is broke.

               2. Sam is broke.

C. Sam did not accept the job. (!!INVALID!!)

Again, it is easy to see why both arguments are invalid: Sam might buy a new car even without accepting the job, and Sam might be broke even though he did accept the job. Nonetheless, most of us feel like there is some persuasiveness to these abductive arguments, and that is because the conclusion would explain the premises. We can reconstruct them as follows:

               1.  If Sam accepted the job, then Sam will buy a new car.

               2. Sam will not buy a new car. (Hypothesis)

               C. Sam did not accept the job. (Fact to be explained) (1, 2 MT)

Again:

               1. Either Sam accepted the job, or Sam is broke.

               2. Sam did not accept the job. (Hypothesis)

C. Sam is broke. (Fact to be explained) (1, 2 DS)

Reconstructing the argument helps us see why these are good hypotheses: if they were true, they would allow us to conclude some fact we know which needs to be explained. At the same time, it also can help us recognize that there are many possible ways to derive the same conclusion from different premises, and so there are often many possible explanations which we need to choose from. How do we weigh competing hypotheses? We’ll turn to that question in the next two pages.

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12.3.2 Disanalogies and Exceptions

Avoid the fallacy of False Analogy, and don’t compare apples to oranges.

The Plausibility of a Hypothesis

Explanations rely on our background knowledge of the world, much of which relies on inductive generalizations. Inductive generalizations have possible exceptions, however. One way to evaluate a purported explanation as more or less plausible, then, is to see if there are any reasons to think that a particular case may be different or atypical, an exception. For example, suppose we want to explain why Sarah broke up with Chuck, and we form the hypothesis that Sarah was jealous:

               1. If Sarah was jealous, then Sarah broke up with Chuck.

               2. Sarah was jealous. (Hypothesis)

               C. Sarah broke up with Chuck. (Fact to be explained)

This takes for granted some background knowledge that jealousy is a common source of break-ups. But, while jealousy might be a cause of break-ups in general, unless we know something further about Sarah and Chuck’s relationship, we don’t have enough reason to think that Sarah was jealous. To evaluate the strength of our hypothesis, its degree of plausibility, we would want to know of any similarities or dissimilarities between Sarah and Chuck’s relationship, and other relationships which broke up due to jealousy.

For instance, if Sarah and Chuck frequently argued due to Chuck’s having an active profile on an online dating site, and other relationships which broke up due to jealousy, often involved active profiles on online dating sites, then the hypothesis is more plausible. Or, if many couples who broke up due to jealousy involved an ex who lived in the same apartment complex, and Chuck’s ex-girlfriend lived in the same apartment complex, then the hypothesis is more plausible, though still far from certain. On the other hand, if Sarah and Chuck have had a relationship which is very different from other cases of break-ups due to jealousy, this makes the explanation less plausible. For instance, if Sarah and Chuck have had an open relationship, in which other partners were always welcome, and this arrangement had lasted over a decade, then this likely makes them different from other couples who broke up due to jealousy, and the explanation is less plausible, though still not impossible

Identifying Dissimilarities

So, one task when evaluating a hypothesis is to attempt to identifying dissimilarities between the particular case we’re considering, and the case where the explanation typically holds. For instance, suppose that a police officer shoots an unarmed suspect, and we want to explain why this happened. One common causal influence on police shootings are unconscious racial stereotypes: under stressful conditions, people are more likely to misperceive a harmless tool as a weapon, or to misperceive a blank expression as a threatening expression, when this fits with racial stereotypes, even if the person does not hold any explicit racist attitudes. So, without further information, this might be an explanation for the shooting. Suppose, however, that we then learn that the officer and the suspect were of the same race, and that they knew one another, because they were ex-lovers. Now, the hypothesis of a racially-influenced incident seems less plausible, because this situation seems different from a more common situation we are familiar with.

A common way in which abductive reasoning fails is that the similarities or dissimilarities which we intuitively latch on to, the ones which are the most prominent and significant to us, are not the similarities or dissimilarities which make a difference to what happens in the real world. For instance, a very hot summer in Norway, the extinction of an obscure species of fish in the Pacific, and flooding in Bangladesh do not seem very similar to us, and so we are unlikely to think of them as being related or having a common cause; nonetheless, climate change could offer an explanation for both events. On the other hand, a mass shooting by a middle-aged man, and a mass shooting by a high school student, will seem similar to us because of their similarly terrible effects as mass shootings, but the specific causes and motives of each may make them very different, so that it is impossible to come up with anything which explains them both. We often neglect to recognize a counteranalogy, a case in which the similarities between A and B are outweighed by As similarities to C, and B’s similarity to D. For instance, the motives behind a mass shooting by a middle-aged man may have more in common with another middle-aged man’s choice to murder his family, while the motives behind a mass shooting by a high school student may have more in common with another high school student’s choice to pull out a knife in class and scream obscenities, even though the effects in each case are different. For this reason, it’s important to look for similarities and dissimilarities that link up with consistent, repeated patterns we inductively observe in the world around us, rather than the ones that seem most striking from our point of view.

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12.3.3 Inference to the Best Explanation

When faced with competing explanations, let the best one win.

Inference to the Best Explanation

Explanations help us to understand why something happened, not simply convince us that something happened. However, there is a common kind of inductive argument that takes the best explanation of why x occurred as an argument for the claim that x occurred.

For example, suppose that your car window is broken and your iPod (which you left visible in the front seat) is missing. The immediate inference you would probably make is that someone broke the window of your car and stole your iPod. What makes this a reasonable inference? What makes it a reasonable inference is that this explanation explains all the relevant facts (broken window, missing iPod) and does so better than any other competing explanation. In this case, it is perhaps possible that a stray baseball broke your window, but since (let us suppose) there is no baseball diamond close by, and people don’t play catch in the parking garage you are parked in, this seems unlikely. Moreover, the baseball scenario doesn’t explain why the iPod is gone.

Of course, it could be that some inanimate object broke your window and then someone saw the iPod and took it. Or perhaps a dog jumped into the window that was broken by a stray baseball and ate your iPod. These are all possibilities, but they are remote and thus much less likely explanations of the facts at hand. The much better explanation is that a thief both broke the window and took the iPod. This explanation explains all the relevant facts in a simple way (i.e., it was the thief responsible for both things) and this kind of thing is (unfortunately) not uncommon—it happens to other people at other times and places. The baseball-dog scenario is not as plausible because it doesn’t happen in contexts like this one (i.e., in a parking garage) nearly as often and it is not as simple (i.e., we need to posit two different events that are unconnected to each other—stray baseball, stray dog—rather than just one—the thief).

Inference to the best explanation is a form of inductive argument whose premises are a set of observed facts, a hypothesis that explains those observed facts, and a comparison of competing explanations, and whose conclusion is that the hypothesis is true. The example we’ve just been discussing is an inference to the best explanation. Here is its form:

1. Observed facts: Your car window is broken and your iPod is gone.

2. Explanation: The hypothesis that a thief broke the window and stole your iPod provides a reasonable explanation of the observed facts.

3. Comparison: No other hypothesis provides as reasonable an explanation.

4. Conclusion: Therefore, a thief broke your car window and stole your iPod.

Notice that this is an inductive argument, because the premises could all be true and yet the conclusion false. Just because something is reasonable, doesn’t mean it is true. After all, sometimes things happen in the world that defy our reason. So perhaps the baseball-dog hypothesis was actually true. In that case, the premises of the argument would still be true (after all, the thief hypothesis is still more reasonable than the baseball-dog hypothesis) and yet the conclusion would be false. But the fact that the argument is not a deductive argument isn’t a defect of the argument, because inference to the best explanation arguments are not intended to be deductive arguments, but inductive arguments. As we saw earlier, inductive arguments can be strong even if the premises don’t entail the conclusion. That isn’t a defect of an inductive argument, it is simply a definition of what an inductive argument is!

The Seven Explanatory Virtues

As we’ve seen, in order to make a strong inference to the best explanation, the favored explanation must be the best (or the most reasonable). What makes one explanation more reasonable than another? There are certain conditions that any good explanation must meet. The more of these conditions are met, the better the explanation. The seven explanatory virtues are:

1. Explanatoriness: Explanations must explain all the observed facts. This is the most obvious condition.  For example, if, in order to explain the facts that your car window was broken and your iPod was missing, someone were to say offer the hypothesis that a rock thrown up from a lawnmower broke the window of your car, then this hypothesis wouldn’t account for all the facts because it wouldn’t explain the disappearance of your iPod.

2. Power: Explanations should apply in a range of similar contexts, not just the current situation in which the explanation is being offered. The baseball-dog hypothesis would explain all the observed facts, but it would lack the virtue of “power”, because it doesn’t occur in similar scenarios.

3. Depth: Explanations should not raise more questions than they answer. For instance, it might be that I broke into my own car while sleepwalking, threw my iPod away, and forgot about it. But this would raise even more questions requiring an explanation.

4. Modesty: Explanations should not claim any more than is needed to explain the observed facts. Suppose someone hypothesized that a 24 year old Chinese man with a Tweety Bird tattoo on his left shoulder broke the window of my car and stole the iPod. The problem is that the hypothesis is far more specific than it needs to be in order to explain the relevant observed facts.

5. Simplicity: Explanations that posit fewer entities or processes are preferable to explanations that posit more entities or processes. All other things being equal, the simplest explanation is the best. Suppose that all three of our cars in our driveway were broken into one night and that the next morning the passenger’s side rear windows of each car were broken out. If I were to hypothesize that three separate, unrelated thieves at three different times of the night broke into each of the cars, then this would be an explanation that lacks the virtue of simplicity.

6. Conservativeness: Explanations that force us to give up fewer well-established beliefs are better than explanations that force us to give up more well-established beliefs. For instance, it might be that Bigfoot broke into my car and stole the iPod. The trouble is that this would require me to give up some well-established beliefs, like that Bigfoot doesn’t exist, and that even if he did, he wouldn’t be that interested in music.

7. Falsifiability: Explanations should be falsifiable—it must be possible for there to be evidence that would show that the explanation is incorrect.

We will return during the last module in the course to discuss further the virtues of modesty, simplicity, and conservativeness, which are applications of a principle referred to as “Ockham’s razor” after William of Ockham (1287-1347), the medieval philosopher and logician. We will also return in the last module to further discuss the virtues of explanatoriness, power and depth, which are expressions of the intellectual virtue of curiosity.  For now, we will focus on the most difficult concept to understand, falsifiability.

Falsifiability

Falsifiability can be a confusing concept to grasp. How can anything having to do with being false be a virtue of an explanation? We’ll need an example to illustrate why the possibility of being false is actually a necessary condition for any good empirical explanation.

Consider the following explanation. My socks regularly disappear and then sometime reappear in various places in the house. Suppose I were to explain this fact as follows. There is an invisible sock gnome that lives in our house. He steals my socks and sometimes he brings them back and sometimes he doesn’t. This explanation sounds silly and absurd, but how would you show that it is false? It seems that the hypothesis of the sock gnome is designed such that it cannot be shown to be false—it cannot be falsified. The gnome is invisible, so you can never see it do its thing. Since there is no way to observe it, it seems you can never prove nor disprove the existence of the sock gnome. Thus, you can neither confirm nor disconfirm the hypothesis.

But such a hypothesis is a defective hypothesis. Any empirical hypothesis (i.e., a hypothesis that is supposed to explain a set of observed facts) must at least be able to be shown false. The sock gnome hypothesis lacks this virtue—that is, it lacks the explanatory virtue of being falsifiable. In contrast, if I were to hypothesize that our dog, Violet, ate the sock, then this hypothesis is falsifiable. For example, I could perform surgery on Violet and see if I found remnants of a sock. If I didn’t, then I would have shown that the hypothesis is false. If I did, then I would thereby have confirmed the hypothesis. So the “dog ate the sock” hypothesis is falsifiable, and this is a good thing. The difference between a true hypothesis and a false one is simply that the true hypothesis has not yet been shown to be false, whereas the false one has.

Falsifiability requires only that it be possible to show that the hypothesis is false. If we look for evidence that would show that the hypothesis is false, but we won’t find that evidence, then we have confirmed that hypothesis. In contrast, an unfalsifiable hypothesis cannot be confirmed because we cannot specify any evidence that would show it was false, so we can’t try to look for such evidence (which is what a rigorous scientific methodology requires).

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12.3.4 Moral Arguments

How do people give evidence for claims about what is morally right or wrong?

For a video walkthrough, click here.

Most of the evidence we’ve discussed pertains to descriptive claims, about how things are. How do we handle evidence for or against normative claims, about what people should or shouldn’t do?

It’s important to know, because, sometimes, the premises you need to defend are normative. These claims can’t be defended simply by citing observations or statistics, because we can’t validly derive what ought to be from what is.Below are the top 10 kinds of arguments which can be used to argument for moral or normative claims.

The Top Ten Types of Moral Argument

1. Arguments by Analogy.

The simplest moral argument is that, if we accept something else is right or wrong, or good or bad, then we should also accept this about something which is relevantly similar to it.

1. X is wrong

2. Y is like X

C. Y is wrong.

For instance, “Smoking marijuana is like drinking alcohol. We shouldn’t ban drinking alcohol. So, we shouldn’t ban smoking marijuana.” Conversely, “Smoking marijuana is like injecting heroin. We should ban injecting heroin. So, we should ban smoking marijuana.”

Arguments by analogy depend on how much alike the two things actually are, and how relevant those similarities are.

2. Arguments from Function.

Aristotle held that some things have either a natural function which they are for (eyes are for seeing), or an intended function which they are for (chairs are for sitting). According to him, it is good for a thing to fulfill its function well. Good eyes see well, while bad eyes need glasses, and good chairs are comfortable to sit on, and bad chairs break.

For instance, “Sleep makes people reason better, and the function of humans is to reason, so sleep is good for humans”, or “Education makes people reason better, and the function of humans is to reason, so Education is good for humans.”

Arguments from function only establish that a thing is good for the particular subject which has that function, not that it is objectively good. A “good” land mine is one that explodes well, and a “bad” land mine fails to go off when a person steps on it. But a “good” land mine is not an objectively good thing.

3. Arguments from Reciprocal Relationship.

One “function” of humans may be having close relationships of mutual trust with one another. Human relationships seem to be better when they are marked by reciprocity: each person gives in proportion to what they take. We show preference to our friends and family because they show preference to us. We expect certain things because of the relationships we stand in towards others, or because of our membership in a group of people.

For instance, “Ben is my boyfriend, so he should respond to my text messages”, or “Edith has been with this firm for twenty years, so the firm should give her some warning if they are going to lay her off”, or “Amy is a fellow Sigma Kappa, so as her sorority sisters we should support her.”

Again, establishing that something is owed within a relationship is not enough to show that it is morally right. The mafia boss might be your godfather, but that doesn’t make carrying out hits for him okay.

4. Arguments from Authority.

One kind of relationship in which people can stand to one another, and owe one another, is that of a subject to an authority. Somebody’s boss at work is the person who has the authority to tell them what to do. In exchange for payment, an employee owes service. Citizens have a certain obligation to follow the law. There is no particular reason that cars drive on the right side of the road and not the left side, but that is what the law says, and so that is a reason to follow it. Of course, authority has limits: a boss who tells somebody how to live their life off the job, or a government which regulates which side of the bed people sleep on, has probably exceeded its limits. Authority must also be legitimate: a person on the street yelling comands at you isn’t somebody to follow.

For instance, “April 15th is tax day, so I should do my taxes tonight”, or “You should slow down because the speed limit is only 35 here.”

Recall that “Appeal to authority” is a fallacy: that an authority says something is true or right does not thereby make it true or right. So, an argument that something is illegal or prohibited by the authorities, even if the law is legitimate, just gives a reason in favor of why one should do something, but it isn’t definitive if there are stronger reasons not to do it.

5. Arguments from Consistency.

A basic, logical principle which all laws must follow is consistency. It would be inconsistent, for instance, to require people to drive on both the left and right side of the road. In some sense, the philosopher Immanuel Kant argued, we all exercise authority over our own choices, we issue “commands” or “laws” to ourselves. So, our choices need to be consistent. It would be inconsistent to steal, for instance, because in taking something and then holding on to it I’m simultaneously denying property rights (by taking) and asserting property rights (by holding on). Acting on an inconsistent principle is morally wrong.

For instance, “It’s wrong to break a promise, because if everybody broke promises, there would be no such thing as promises to break, but that would be inconsistent,” or “It’s wrong to pay your employees different amounts based only on their gender, because that is inconsistent.”

While it’s not so clear that all of morality can be derived in this way, certain basic principles (don’t lie, don’t murder, don’t cheat) do seem to be things we can argue against on the basis of consistency.

6. Arguments from Humanity.

All of us reading this are thinking human beings who want things and make choices. According to Kant, that means that we owe ourselves a certain kind of respect, as things with intrinsic value in ourselves. We are not merely a means for somebody else to get what they want. But one thing that would be profoundly inconsistent would be for us to fail to give that same respect to the humanity of other human beings. It is wrong to do something if it treats other humans as merely a means of getting what we want, and not as beings with value in themselves.

For instance, “You should still tip when you stop at a restaurant while driving across the country, out of human respect for the person who gave you food,” or “It is wrong to torture somebody, even if they are a terrible person, because they are still a person.”

While arguments from humanity help bridge the boundaries that normally divide people, they don’t readily give us answers as to how to resolve competing interests between humans.

7. Arguments from Justice

Imagine that all human beings were in a kind of idealized situation, looking at the world before any of us were born, not knowing where or when we’d be born or what station in life we would have or what circumstances we would be born into.  The philosopher John Rawls asks: what kinds of moral rules or laws would we pass together to govern our world? This can be a helpful way of thinking about justice. Justice is the set of rules that we would want ideally to govern the world. The burdens and benefits in a society should be distributed fairly, and those who wrongly take from others repay what they took.

For instance, “We should accomodate those with disabilities, because we might have had or someday will have a disability”, or “We should take in refugees, because we might have found ourselves refugees.”

8. Arguments from Consequences.

One rule which we might all agree on in an idealized situation is that, given a choice between two options, we ought to pick the better of the two options: the option which brings about more benefit or less harm. This does not mean, as many people wrongly think, that it is always wrong to bring about harm, and always right to bring about something beneficial. Rather, it can be right to bring about the least harmful option, and wrong to do something beneficial if there is an even more beneficial alternative.

For instance, “You shouldn’t spend your time doing volunteer work when you could make money and donate it to charity instead”, or “You should have the surgery, because even though it is painful, it is better than dying.”

What is “better” or “worse” is a question about value. What is valuable? What do we value? That is a difficult question.

Avoid the “Good to Better”, “Better to Best” and “Bad to Worse” Fallacies!

• Just because one option A is good dooes not make it better than other options when all things are considered.
• “This job pays $25/hr, it’s interesting, and isn’t too hard. Therefore, it is better than a job which pays $30/hr, is more interesting, and even less hard.”
• Just because one option A is better than another option B doesn’t mean that A is the best option there is.
• “It would be better for me to work as a lawyer than work as a blacksmith. Therefore, working as a lawyer is the best job for me.”
• Just because one option is baddoes not make it worse than other options.
• “Losing your job is bad. Therefore, losing your job is the worst thing that could happen to you.”

9. Arguments from Freedom.

To say we value something is to say we’d choose it over some less valuable alternative. So, of all the things we value, one thing we have to value is our own choice: our own freedom to choose one thing over another. If our ability to choose had no value, then it wouldn’t matter what we chose, and nothing would have value. So, one kind of moral argument people make is that something is wrong because it restricts our freedom to make choices, or right because it enhances it.

For instance, “Arranging your child’s marriage for them is wrong, because love which isn’t a free choice isn’t authentic,” or “Forcing students to salute the flag is wrong, because patriotism should be a choice.”

While freedom may be a good, however, it should be obvious that not every choice which somebody makes with their freedom is thereby a good choice.

10. Arguments from Responsibility.

Everyone is responsible for how they exercise their freedom. If a person exercises their freedom to choose the worst choice, or to do something wrong, then others will hold them responsible for their choices, and responsible to repay or recompense the damage they cause. Even if a person does no wrong, they can choose to adopt or take up responsibility for something which they were not previously obliged to do, like deciding to take responsibility for the mess in the shared kitchen of a dormitory. When somebody regards themselves as responsible for something, then they should do it.

For instance, “You offended her, so it is your responsibility to apologise, and you should call her,” or “If nobody is responsible for sorting these books, then they’ll never be sorted, so I have chosen to be responsible for them, and I should sort them.”

Conclusion

There are other kinds of moral arguments out there, and many variations of the but these 10 types of arguments cover most of the moral or normative arguments you are likely to encounter.

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Submodule 12.3 Quiz

Key Sources:

• Watson, Jeffrey (2019). Introduction to Logic. Licensed under: (CC BY-SA).
• Modified from Introduction to Logic and Critical Thinking by Matthew Van Cleave, pp 146-150, licensed under a Creative Commons Attribution 4.0 International License.
• Incommensurable  by Robert Couse-Baker, under license CC BY-SA 2.0,  via Flickr
• DUI/DWI Sobriety Test – Walking the Line by SanDiego DUIAttorney under license CC BY-SA 2.0, via Flickr.

Next Page: 13.1 Weighing the Evidence