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Before we dive into the big questions of philosophy, you need to know how to argue properly. We’ll start with an overview of philosophical reasoning and breakdown of how deductive arguments work (and sometimes don’t work).


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Hank: Aristotle once described humans as “the rational animal.” Well, actually, he said that “man is the rational animal,” but we don’t have to be sexist just because he was. And if you’ve ever gotten into an argument with someone about religion or politics or which Hemsworth is the hottest, then you’ve experienced how irrational people can be about their opinions.

But what Aristotle meant is that rationality is our distinguishing characteristic – it’s what sets us apart from the beasts. And no matter how much you disagree with someone about God or Obama or Chris Hemsworth, you can at least grant that they are not beasts. Because, most of the time at least, people can be persuaded. By arguments.

You use arguments all the time -- in the comments, at family dinners, with your friends -- you probably just don’t think of them the same way that philosophers do. When you try and convince your parents to loan you the car, or when you’re talking up Crash Course to your friends, you are using arguments. Thanks, by the way.

Each time you tell someone to do or believe something -- or when you’re explaining why you do or believe something -- you are giving an argument. The problem is, the vast majority of people aren’t really good at arguments. We tend to confuse making a good argument with, like, having witty comebacks, or just making your points more loudly and angrily, instead of building a case on a solid foundation of logic. Which can be harder than it sounds.

But learning about arguments and strong reasoning will not only make you a better philosopher, it will also set you up to be a more persuasive person. Someone who people will listen to. Someone who’s convincing. So, yeah, these skills are beneficial no matter what you want to do with your life. So you might as well know how to argue properly.
[Theme Music]

 The Value of Reason

If you want to learn how to argue, then you should probably start about 2400 years ago, when Plato was laying out how reason can, and should, function in the human mind. He believed that we all have what he called a tripartite soul – what you might think of as your “self,” or your psyche, divided into three parts.

First, there’s the rational, or logical part of the soul, which represents cool reason. This is the aspect of your self that seeks the truth and is swayed by facts and arguments. When you decide to stop eating bacon for two meals a day because, as delicious as it is it’s bad for you, then you make that decision with the guidance of the rational part of your soul.

But then there’s the spirited aspect, often described as the emotional part of the self, although that doesn’t really quite capture it. The spirited soul isn’t just about feeling -- it’s also about how your feelings fuel your actions. It’s the part that responds in righteous anger at injustice, the part that drives your ambition, and calls upon you to protect others. It gives you a sense of honor and duty, and is swayed by sympathy. So if you decide to stop eating bacon because you just finished reading Charlotte’s Web, and now you’re in love with Wilbur, then you’re being guided by the spirited part of your soul.

But we share the next part of our soul with other animals, be they pig, or moose, or aardvark. The appetitive part is what drives you to eat, have sex, and protect yourself from danger. It is swayed by temptations that are carnal, and visceral. So at those times when you go ahead and just EAT ALL THE BACON because it just smells so dang good, the appetitive aspect of your soul is in control.

Now, Plato believed that the best human beings -- and I should point out here that Plato most definitely did believe that some people were better than others -- are always ruled by the rational part of their soul, because it works to keep the spirited and the appetitive parts in check. People who allow themselves to be ruled by their spirited or appetitive selves are base, he believed, and not fully, properly human.

Now, most of us don’t buy into the concept of the tripartite soul anymore -- or the idea that some humans are less human than others. But we do understand that we’re all motivated by physical desires, emotional impulses, and rational arguments. And philosophers continue to agree with Plato that reason should be in the driver’s seat. So, how do you know if you’re good at it? How can you test your reasoning? Well, let’s head over to the Thought Bubble for some Flash Philosophy.

Throughout this course, we’re going to apply our philosophical skills by pondering puzzles, paradoxes, and thought experiments. Because remember: Philosophers love thinking about questions -- especially ones that don’t have ready answers. So think of these exercises as philosophical wind-sprints -- quick tests of your mental abilities.

And here’s a doozy, from 20th century British thinker Bertrand Russell, one of the pioneers of what’s known as analytic philosophy. Say there’s a town in which all men are required by law to be clean-shaven. This town has only one barber, a man, who must follow strict rules: Rule number one: He must shave all men who do not shave themselves. Rule number two: He must not shave any man who does shave himself. It’s the nightmare of every libertarian and every mustachio’d hipster. But here’s the question: Does the barber shave himself?

Cause think about it: The barber only shaves men who don’t shave themselves. So if he does shave himself, then he must not, because the barber’s not allowed to shave guys who shave themselves. But, if he doesn’t shave himself, then he has to be shaved by the barber, because that’s the law.

Russell came up with this puzzle to illustrate the fact that a group must always be a member of itself. That means, in this case, that “all men who shave themselves” has to include every guy who shaves himself, including the barber. Otherwise, the logic that dictates the group’s existence just doesn’t hold up. And if the barber is a logical impossibility, then he can’t exist, which means the reasoning behind his existence is inherently flawed. And philosophy doesn’t tolerate flawed reasoning.

 Structure of Arguments and a Close Look at Deductive Reasoning

So, how do we make sure that we’re ruled by good, sound, not-flawed reason? By perfecting the art of the argument. An argument, in philosophy, isn’t just a shouting match. Instead, philosophers maintain that your beliefs should always be backed up by reasons, which we call premises. Premises form the structure of your argument. They offer evidence for your belief, and you can have as many premises as you like, as long as they support your conclusion, which is the thing that you actually believe.

So, let’s dissect the anatomy of an argument. There are actually several different species of arguments. Probably the most familiar, and the easiest to carry out, is the deductive argument. The main rule of a deductive arguments is: if your premises are true, then your conclusion must be true. And knowing that something is actually true is very rare, and awesome.

So, here’s a boiled-down version of a good deductive argument: Premise 1: All humans are mortal. Premise 2: Socrates is a human. Conclusion: Socrates is mortal. This kind of reasoning, where one fact leads to another, is called entailment. Once we know that all humans are mortal, and that Socrates is a human, those facts entail that Socrates is mortal. Deduction begins with the general – in this case, what we know about human mortality – and reasons down to the specific – Socrates in particular. What’s great about deductive arguments is that the truth of the premises must lead to the truth of the conclusion. When this happens, we say that the argument is valid – there’s just no way for the conclusion to be false if the premises are true.

Now check out this argument: All humans are mortal. Socrates is a human. Therefore, Socrates was Plato’s teacher That argument is invalid, because nothing about human mortality can prove that Socrates was Plato’s teacher. As you might have noticed, there are plenty of mortal humans who never taught Plato. What’s interesting, though, is that this argument does happen to have a true conclusion, which leads us to another issue. And that is: Validity is not the same as truth. All ‘valid’ really means is that if the premises are true, then your conclusion can’t be false. But that doesn’t mean that your premises prove your conclusion to be correct. Like, in the case of whether Socrates was Plato’s teacher, the premises are true, and the conclusion is true, but the argument is still not valid -- because the premises don’t in any way prove the conclusion. It just happens to be true.

So, if your premises don’t guarantee the truth of your conclusion, then you can end up with some really crappy arguments. Like this one: - All cats are mammals - I’m a mammal - Therefore, I’m a cat As much as part of me would like to be my cat, this is invalid because the conclusion doesn’t entail from the premises... at all. I mean, all cats are mammals, but all mammals aren’t cats. Which means there are such things as non-cat mammals, which I am just one example of. And it probably goes without saying, but you can have a perfectly valid argument and still have a false conclusion, if any of your premises are false.

For example: - All humans have tails - My brother John is a human - Therefore, John Green has a tail! The argument is totally valid! – Because the premises entail the conclusion! The reasoning totally stands up! It’s just that one of the premises is flawed. Since I’m reasonably certain that John doesn’t have a tail -- I’ve seen him in a bathing suit -- this argument is not deductively sound. And a deductively sound argument is one that’s free of formal flaws or defects. It’s an argument whose premises are all true, and that’s valid, which means its conclusion is guaranteed to be true. So, sound arguments should always be your goal.

The reason that deduction is prized by philosophers -- and lots of other important kinds of thinkers -- is that it’s the only kind of argument that can give you a real certainty. But it’s limited, because it only works if you’re starting with known, true premises, which are hard to come by. And for what it’s worth, deductive truths are usually pretty obvious. They don’t tend to lead us to startlingly new information, like the fact that I’m not a cat, or that John doesn’t have a tail. So instead of starting with premises that are already certain, like deduction does, you’re gonna have to know how to determine the truth of, and your confidence in, your premises. Which means you’re going to have to acquaint yourself with the other species of arguments, which we’re gonna do next time.


But today, we talked about the value of reason, the structure of arguments, and we took a close look at one kind of argument: deductive reasoning. This episode of Crash Course Philosophy is made possible by Squarespace. Squarespace is a way to create a website, blog or online store for you and your ideas. Squarespace features a user-friendly interface, custom templates and 24/7 customer support. Try Squarespace at for a special offer.

Crash Course Philosophy is produced in association with PBS Digital Studios. You can head over to their channel to check out amazing shows like The Art Assignment, The Chatterbox, and Blank on Blank. This episode of Crash Course was filmed in the Doctor Cheryl C. Kinney Crash Course Studio with the help of all of these amazing people and our Graphics Team is Thought Cafe.