For several centuries there have been hairless dogs in western Mexico and in coastal Peru. It is very unlikely that a...

Daniel on February 17 at 05:13AM

Why not A?

Wouldn't a negation of A also make this argument fall apart? If the hairless dogs were found elsewhere in the world, then we can't say the dogs must have been traded between Mexico and Peru. Please explain, thanks!

5 Replies

on February 18 at 06:27PM

Hello @dannyod,

You may have a point that the negation of A does weaken the conclusion. However, weakening is not enough. We need the argument to fall apart completely. I'll explain why negating A does not do so.

Negation: Hairless dogs have been found in places other than western Mexico and coastal Peru.

Even if this is true, is it still possible that the dogs were brought to Mexico from Peru or vice versa. Maybe the dogs were introduced to other places centuries later. The argument can still stand.

Now let's try negating E.

Negation: Centuries ago, it was not easier to travel by boat between Mexico and Peru than by an overland route.

The author's conclusion is based on the idea that overland travel is extremely difficult, so it must have been by water. If we eliminate that premise, then the argument falls apart. The author has no reason to conclude that the dogs were bought by boat, when it was just as easy, if not easier, to bring them by land.

Daniel on February 24 at 01:17AM

Thanks for clarifying, but I am still confused. Isn't the author's conclusion that the dogs MUST have been traded from Mexico to Peru via boat. Answer A, in my view, degrades the conclusion because it no longer MUST be true (I.e. there is a legitimate alternative so no single explanation must be true). Sorry if this is nit picky, I'm just trying my best to understand. Why is the overwater v overland piece what we are paying attention to in this conclusion, vs the fact that it must have been traded between these two countries?

Daniel on March 21 at 08:24PM

Bumping up for clarification, thanks!

Daniel on April 8 at 03:12PM

@samA are you or another instructor able to clarify?

jing jing on September 6 at 08:34PM

Hi I am not an instructor but I understand why you are confused since this is a tricky question. Our instructor’s answers are helpful to me in confirming my understanding of this problem so thank you to the instructors for answering the questions.

Premise 1: for centuries hairless dogs exist in Mexico and in coastal Peru
Premise 2: This trait of hairlessness emerging on two separate occasions such as in coastal Peru and in Western Mexico is very unlikely to happen.
Premise 3: Dogs were never in the wild.
Premise 4: vast mountainous jungle separating these two regions namely Mexico and coastal Peru would have made overland travel between them extremely difficult centuries ago
Conclusion: the dogs must have been transported from one of these regions to the other by boat

A is incorrect because its negation doesn’t contradict any of the premises. Premise 1 simply states hairless dogs exist in Mexico and in Coastal Peru. Premise 1 doesn’t say hairless dogs cannot be found elsewhere other than Western Mexico and Coastal Peru. It could be the case that hairless dogs were introduced to other places later on after they have been transported between Mexico and Peru. Thus, A cannot be an assumption since we don’t need to establish its truth to make the argument valid. The negation of A, namely that hairless dogs are found elsewhere other than Western Mexico and Coastal Peru, makes the argument perfectly valid and does not contradict any of the premises.

E, however, is an assumption we need to make the argument valid. The negation of E, namely that it was harder to travel by boat between Mexico and Peru than to travel by overland route, would directly contradict Premise 4, which states that “vast mountainous jungle separating these two regions namely Mexico and coastal Peru would have made overland travel between them extremely difficult centuries ago.” Since E is needed to support Premise 4, E must be an assumption that the argument relies on.

I always find it easier to break an argument into premises and conclusion if I am stuck on the answer choices. This is my interpretation of the question. I only became more confident in my interpretation after I read our instructors’ answers which seem to confirm my understanding of this problem. Please correct me if I am wrong. Thank you for your questions and answers.