About this video
This LSAT-style lesson teaches how to match the flawed reasoning pattern in an argument. The original argument says a book tour succeeds if it is well-publicized and the author is established, then infers from success plus an established author that the tour was well-publicized. The flaw is affirming one sufficient condition as if it were the only way. Answer choices that use only if, which makes the condition necessary, are ruled out, and the correct choice is C.
You'll be able to
- identify the flawed reasoning pattern of affirming one of several sufficient conditions.
- distinguish if from only if in conditional statements.
- eliminate answer choices that share a surface structure but not the same flaw.
Key words
- Sufficient condition
- A condition that, if met, guarantees the outcome, though other routes may also lead to it.
- Necessary condition
- A condition that must be met for the outcome to happen.
- Only if
- A phrase that makes the stated condition the sole way for the outcome to occur.
- Parallel reasoning
- Choosing an argument that has the same logical structure and flaw as the original.
Transcript
- [Instructor] Let's see, we're told a book tour will be successful if it is well-publicized and the author is an established writer. Julia is an established writer, and her book tour was successful. So her book tour must have been well-publicized. Let's just think about what they're saying.
They're saying a book tour will be successful, so successful... Successful tour. and that's going to happen if it is both well-publicized, well-publicized, and the author is an established writer, plus established writer. So the way it's set up, they're saying "Hey, if both of these things happen "then you will have a successful tour."
But then they go on to say Julia is an established writer and they say her book tour was successful so therefore her book tour must have been well-publicized. Well that doesn't necessarily fall out of the original statements, the original statements don't say this is the only way that you're going to have a successful tour, if this was the only way to have a successful tour, then maybe this last statement might be reasonable.
But they don't say this is the only way, they say this is a way of having a successful tour, by have being well-publicized and by being an established writer. So let's read the question: Which of the following exhibits a pattern of flawed reasoning most closely parallel to the pattern of flawed reasoning exhibited by the argument above?
So statement A says: This recipe will turn out only if one follows it exactly and uses high-quality ingredients. So the recipe works so I'll call it, they say turn out, the recipe turns out only if follows exactly, follows exact, and high-quality ingredients, high-quality ingredients.
So if you do both of these things it is going to turn out. Arthur followed the recipe exactly, so he followed it exactly, and it turned out. Thus, Arthur must have used high-quality ingredients. So this one looks similar to what we did there where they're saying "Okay, it turned out "and he followed it exactly, "and so he must have used high-quality ingredients."
But the difference here is they're saying this recipe will turn out, if and only if, will turn out only, or I should say this one says this recipe will turn out only if, so it's saying this is the only way to get it to turn out. So if it turns out, then both of these two things must be true, and so it's reasonable to say that Arthur must have used high-quality ingredients.
Over here, they didn't say a book tour will be successful only if, if that was true then the argument would not have been flawed. So I'll rule this one out, even though it looks similar the difference here is they're saying only if, this is the only way to get the recipe to turn out, while over here they're not saying this is the only way to have a successful book tour.
Choice B: If a computer has the fastest microprocessor and the most memory available, it will meet Aletha's needs this year, so if it's fast plus memory, plus most memory, so if you have both of those it will meet Aletha's needs, it will meet Aletha's needs. This computer met Aletha's needs last year.
So it must have had the fastest microprocessor and the most memory available. So this one is different than our original statement, because in the original statement we said "Okay, we got a successful tour "and she's an established writer." And then they picked out one the others conditions, or one of the other ways to get a successful tour.
Here they're saying "Hey, the conclusion--" or "We were able to meet the needs." And then they're saying "Therefore, both of the inputs "must have happened." So this is a little bit different, so I would rule this out. If cacti-- Let me scroll all the way. If cacti are kept in the shade and watered more than twice weekly, they will die.
So shade plus two times watering, they will die. This cactus was kept in the shade, and is now dead. Therefore, it must have been watered more than twice weekly. So this one looks really interesting, because just looking at the argument doesn't necessarily mean that it must have been watered twice more than twice weekly, it doesn't say that cactus will die only if they are kept in the shade and they are watered more than twice weekly.
If it was only if, like we saw in choice A, well then you might be able to make that conclusion but this is very similar to the original statement where they're saying "Hey look, if this and this happens, "then this is going to happen." And then they give a situation where they say "Okay, it did die, and one of the things happened."
And so they're trying to make the conclusion that the other input, that other condition must have happened, but that's not necessarily the case. It would only be the case if they said "The only way to kill the cactus is if they "are kept in the shade and watered more than twice."
So this one seems to have the exact same structure as our original argument, the same flaw I should say. So let's just verify the last two choices: A house will suffer from dry rot and poor drainage only if it is built near a high water table, so built high water table, so if it is built near a high water table it will suffer dry rot plus poor drainage, poor drainage.
This house suffers from dry rot and has poor drainage. Thus, it must have been built near a high water table. Well the way this is different is actually-- only if it is built near a high water table, so this one is actually a reasonable statement to say 'cause they say only if it is built near a high water table this will happen.
So this is happening, it's actually reasonable to say that it was built near a high water table. So not only is this not flawed like our original statement but also has a different structure, it's not saying "Hey, there's two things that need to be true--" Or "There's two things if they were true "would make the last statement true."
So it has a different structure so I'd rule this one out. And this last choice: If one wears a suit that has double vents and narrow lapels, so double vents plus narrow lapels, narrow lapels, one will be fashionably dressed. So then you will be fashionable, I wish it were that easy.
The suit Joseph wore to dinner last night had double vents.. and narrow lapels, so Joseph must have been fashionably dressed. So this one, if you assume this original part if you say "Hey, if one wears a suit that has double vents "and narrow lapels, one will be fashionable dressed."
If you accept that conditional, if you said this is true, then actually the conclusion is fairly reasonable because Joseph is meeting those first two conditions and so if you believe them, then you could say that he must have been fashionably dressed. So this is a-- one, it's not flawed the way the original argument was, and it also, fundamentally, has a different structure so I feel good about choice C.
Video © Khan Academy, used under CC BY-NC-SA. Watch on Khan Academy. Puzzicle is not affiliated with Khan Academy.