Conditional Reasoning: Assumption Questions

[cmd14cd]

Confused about page 363 in the LR 2017 Edition book.

It says
P: A --> B --> C
C: A --> D
Assumption C--> D

My question is how do you know that its not D-->C, or the D-->B, theoretically we only know about A, not about the bigger groups of B or C, how can you assume B--> C--> D?

For example take the sentence to match the above structure:
P: If you're a Laker, then you're in the NBA, then you're a professional athlete.
C: Thus, if you're a Laker, then you're male.
The assumption that if you are a professional athlete then you are a male is not correct.

The reason I am asking this is sometimes there are questions that are about obscure subjects that I am not familiar with and its easy to get confused by the wording. I just don't understand how you can assume that C-->D?

[Jonathan Evans]

Hi, CMD,

Good questions.

In this hypothetical scenario, the conditional chain A B C is provided as a premise. In other words, you do know already that A implies B and B implies C. You may infer correctly just from the premises that A implies C (A C).

In the conclusion, the author attempts to infer that A implies D (A D), but there is no existing link. The implicit assumption here is usually that C implies D (C D) because that would enable you to complete the chain:

• A B C D
  A D

It is possible that the assumption could be B D because then we can disregard C and jump straight from A to B to D:

• A B D
  A D

The example included in the book illustrates a common scenario, perhaps the most common scenario, but it is not the only possible assumption we might encounter on a problem such as this.

However, the assumption is not D C and also is not D B. Both of these would be Mistaken Reversals™ and would not complete the conditional chain correctly.

For example take the sentence to match the above structure:
P: If you're a Laker, then you're in the NBA, then you're a professional athlete.
C: Thus, if you're a Laker, then you're male.
The assumption that if you are a professional athlete then you are a male is not correct.

Good example! You are correct that the assumption is not correct (in real life anyways), but this assumption (if you are a professional athlete, then you are male) could be a valid expression of an implicit assumption the author makes in his or her argument. This assumption would enable you to complete your chain conditional.

In the way the author has chosen to formulate the above argument, "if you're a professional athlete, then you are male" is an assumption. Otherwise, the above argument would not make sense. If being male is not a necessary condition for being a professional athlete, then we would be unable to complete our chain conditional and the argument would fall apart.

Remember that the LSAT is not concerned with real world truth but only with the logical validity of the arguments we encounter based on hypothetical premises.

You are right that this can get confusing and weird at times! A couple examples of such weirdness occur to me right off the top of my head. Just remember to take the facts presented to you at face value and analyze the reasoning present on its merits.

I hope this helps!

[cmd14cd]

Yes thank you very much!