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We continue our look at philosophical reasoning by introducing two more types: induction and abduction. Hank explains their strengths and weaknesses, as well as counterarguments and the Socratic method.


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How do you know that aspirin will take care of your headache? Why do you really wanna see that new Marvel movie, even though you haven't heard anything about it, good or bad? Your ability to do things like predict how a medication will affect you, or what movie you might like, or even things like what the perfect gift might be for your best friend, or what's the fastest way to get to campus, all of this stuff, you know through induction.

Deductive arguments are great because they give us certain answers, but unfortunately much of the world cannot be summed up in a neat deductive proof. Deduction requires a fair amount of general information to give you a specific conclusion that is, frankly, probably kind of obvious. 

So philosophy, and basically, y'know, life as well, require that you have other ways of reasoning. In addition to knowing how one fact leads to another, you also need to take what you've experienced before and use that to predict what might happen in the future. And you need to be able to rule out what can't be true, so you can focus on what can.

Through these kinds of reasoning you're not only able to figure out stuff like how to fix your headache and why your roommate might be acting weird, you can also come up with better, more skillful arguments and counterarguments, which are some of the most important maneuvers in the philosophical game.

And maybe the best part is, you already know how to use these techniques. In fact, I bet you used them this very day. You know this.

(Theme Music)


If you possess any ability to really predict the future, it lies in your ability to reason inductively. Inductive reasoning relies on the predictability of nature to reveal that the future is likely to resemble the past, often in important ways.

For example, there's tons of research to support the knowledge that aspirin, acetylsalicylic acid, is an effective treatment for pain, like headaches. And you probably have personal experience with the effects of aspirin, too. So you believe that this aspirin tablet will cure the headache you have right now because countless aspirin tablets have cured countless headaches in the past. Likewise, you wanna see the new Marvel movie because you liked most of the other ones, so you believe that they'll continue to deliver for you, entertainment-wise.

But it's important to remember that unlike deduction, where true premises entail true conclusions, inductive premises only mean that the conclusion is likely to be true. Inductive arguments don't provide you with certainty. Instead, they work in terms of probabilities. And they're useful for more than just predicting what's going to happen, for example:

- Most men in ancient Athens had beards.
- Socrates was a man who lived in ancient Athens.
- Therefore, Socrates probably had a beard.

This is an inductive argument because it starts with what we already know about the grooming habits of ancient Athenian men, and about the time and place in which Socrates lived, and makes an educated guess based on that information. There's no guarantee that the conclusion is correct, but what's known would seem to support it.

Reasoning like this is incredibly useful, which is why it's so common. But, there's also a problem. The future doesn't always resemble the past, and every pattern has its outliers. So induction always has the potential to produce false results; aspirin might not work on a really bad headache, the new Marvel movie might be awful, and, yeah, maybe a specific guy in Athens had a beard, but it's possible he didn't.

While the world tends to work according to predictable rules, sometimes those rules are violated. And you know what you need when that happens: a little flash philosophy. Off to the thought bubble.

Contemporary American philosopher Nelson Goodman confronts the problems with induction using a thought exercise about a hypothetical substance called grue. According to Goodman's scenario, grue is anything that's the color green before a certain time, a time we will call t. And another property of grue is that while it's green before time t, it's blue after it. 

Now let's assume that we're living in a time before t. t could happen a hundred years from now or tomorrow, but we know that all the emeralds we've ever seen are green. So inductive reasoning lets us conclude that all emeralds are green and will remain green after time t since emeralds have never been known to change color. But all emeralds are grue because it's not yet time t and they're green, which is part of the definition of grue. So we have no choice but to conclude that emeralds will be blue after time t arrives.

Now we've got a problem because inductive reasoning has lead us to conclude that emeralds will be blue after time t, but inductive reasoning also tells us that they'll remain green. Goodman's riddle reminds us that inductive evidence can be flawed or contradictory. It can make you think that you can predict the future when, of course, you can't.


So, there are times when you need to get at the truth in other ways, like by eliminating what's obviously not true and considering what's most likely. And for this, we turn our attention to one of the most important philosophical figures of 19th century England, Sherlock Holmes. 

In chapter six of Sir Arthur Conan Doyle's The Sign of the Four, Mr. Holmes says, and I quote, "when you have eliminated the impossible, whatever remains, however improbable, must be the truth." This is probably the best, most succinct description ever given of the kind of reasoning known as abduction. Which I know sounds like we're talking about kidnapping or something, but abduction is a thought process sometimes described as "inference to the best explanation." 

Abduction doesn't reason straight from a premise to a conclusion, as we've seen in deduction and induction. Instead, it reasons by ruling out possible explanations until you're left with the most plausible one given the evidence. Consider this:

- Anna told you she failed her physics midterm.
- Anna hasn't been in physics class since your teacher graded the exams.
- Anna has been in sociology class, which meets right after physics.
- Anna dropped physics.

Now, with only these premises, we can't deductively or inductively prove our conclusion - that she dropped physics. But it's a justifiable conclusion because given what we know, dropping the class is the most plausible explanation of events. We know she's not sick because she's still going to sociology, and we know she had a good reason to withdraw from the class because she was unlikely to pass. Concluding that she dropped the course makes the most tidy use of our information without leaving any loose ends. So let's look at another one: 

- You and your roommate ate sushi last night.
- You both wake up with violent stomach aches. 
- You and your roommate ate some bad sushi.

The mere fact that you're both sick doesn't prove that the sushi caused the sickness, but given that you both ate the same thing and you both have the same symptoms, absent other information like that a stomach virus is going around your dorm, the best explanation is that the sushi caused your intestinal anguish.

Now, like induction, abduction doesn't give us certainty. But it is a really useful way to get through puzzling situations when you don't have clear evidence from the past to help you out. Doctors use abduction a lot when they're diagnosing illnesses, and detectives use it when piecing together evidence. You probably use it pretty often, too. Just beware, because abduction must be used carefully. It uses only information you have at hand, that's why doctors and detectives work so hard to dig up more data and recreate events from the past, so they can help draw better conclusions.


Alright, now that we've looked at some argument types, let's find out how philosophers use arguments to interact with each other. Because philosophers don't argue like other people do. It's not like the conversation you have around the dinner table about whether the Patriots are better than the Seahawks, or why plain M&M's are superior to peanut, which is clearly a preposterous position to take. Philosophers hold each other to different, higher standards.

They don't teach each other to get away with saying "I reject your argument because I don't like its conclusion," or "that's preposterous, peanut M&M's are so good." Instead, if you disagree with a conclusion, you need to give reasons, just like the first person did when they made their case.

Both people involved in this kind of exchange are known as interlocutors because we have to name everything. The first one advances an argument and the second one can either accept it or offer a counterargument, which is just what it sounds like, an argument offered in opposition to another argument.

Think back to Socrates and the beard. You think Socrates had a beard and your reasoning is most men in his time and place had them. I, however, think you're wrong, so I give you a counterargument. Gorgias, a contemporary of Socrates, said Socrates couldn't grow a beard and that he would sneak into barbershops and steal discarded clippings to fashion fake beards for himself. Therefore Socrates didn't have a (real) beard.

And I just want to point out that this is an actual philosophy conspiracy theory. Gorgias was a real guy who differed with Socrates on many things and the dispute was said to have gotten personal. According to accounts of the time, Gorgias actually spread the rumor that Socrates wore, like, a beard wig in an effort to shame and discredit his rival. I mean, how could you be a good thinker if you weren't a good beard-grower?

Gorgias' gossip didn't go over well with everyone, and in this instance, let's say you are skeptical about it, too. So you counter my counterargument with a counter- counterargument: Gorgias was known for being a gossip and for hating Socrates and trying to make him look bad. His fake beard tale seems wildly unlikely. Therefore, we can't take Gorgias' statement seriously, so we should fall back on the best information we have, which is that most of the men in his time and place had beards. 

And as you can see, arguments of different styles can be used in the same exchange. Like, the original argument about Socrates probably having a beard was inductive, but this last counterargument is abductive, and that's fine. Arguments are meant to be useful, so we don't have to use the same kind of reasoning when we argue. 

 Socratic Method

This way of exchanging ideas through dialogue was popularized by Socrates and so has become known as the Socratic method. Socrates thought dialogue was the best way to learn and to get at truth. And it's important to note that while philosophers have a reputation for being an argumentative lot, they don't think of the Socratic method as something that results in a winner and a loser. Rather, it's an exercise that brings both interlocutors closer to the truth. The goal of the philosopher is not to win, but to find truth, so you shouldn't be disappointed if someone presents a counterargument that you can't find a response to. When that happens, a good philosopher will be grateful to their interlocutor for helping them reject false beliefs and build stronger ones.


Today, you learned about two more types of philosophical reasoning, induction and abduction. You've seen their strengths and their weaknesses, and you've also learned about counterarguments and the Socratic method. 

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Crash Course: Philosophy is produced in association with PBS Digital Studios. You can head over to their channel to check out amazing shows like Brain Craft, It's Okay to Be Smart, and PBS Idea Channel. 

This episode was filmed in the Doctor Cheryl C. Kinney Crash Course Studio with the help of these amazing people and our graphics team is Thought Cafe.