Assumption and conditionality questions

[kggaines]

Mr. Killoran,
I had a question about your description of assumption questions in the Logical Reasoning Bible. From pages 260-273 of the 2013 edition, you go into the details of assumption questions and conditional reasoning. On page 262 and 273, you discuss supporter assumptions and the two different types of assumption answer choices respectively. My question is how do supporter assumptions and the first type of assumption answer choice that you discuss on page 273 differ from justify questions? In your discussion on page 262, you describe supporter assumptions as elements in the stimulus that fill logical gaps in the argument, and then on page 273 you mention that the correct answer choice fills in a missing link in the chain of logic. Isn't this exactly what a justify question does though? I understand the defender assumption type pretty well, since this type of assumption rules out possibilities or other considerations, but I am having a hard time separating the supporter assumption type and a regular old justify question. Problem set question number 1 on page 283 further clarifies my question, since the correct answer choice *seems* like it meets the threshold of what would be sufficient to make the argument a valid one. Hopefully this wasn't too confusing, and thanks very much for your time.

Respectfully,
Kyle

[Dave Killoran]

Hi Kyle,

Thanks for the question! I don't have a 2013 version on hand where I'm at right now, so I can't cite pages for you, but I know the general content (and as an aside, in the current version of the book, the Assumption chapter starts on page 342, so you can see that in just a few short years, I've added a significant amount of new content).

The difference between Justify and Assumption questions comes down to the role they play and consequently what can appear in the correct answer choice. That said, the two do overlap significantly, and often the same answer choice could be correct for either type of problem. So, your instincts here are right on! What then, is the difference?

Think about Assumptions as minimalist elements. The correct answer will be very direct, and not include any extraneous elements. So, if there's anything "extra" in an Assumption answer, that answer will be wrong. The correct answer is literally the bare bones that are needed, and not a speck extra.

Justify questions are entirely different in that respect. You can have "extra" stuff in a Justify answer and it can still be correct. You just need to meet the minimum threshold to justify the conclusion; anything else that's included doesn't take away from meeting that threshold.

To use a simple example, let's say you have the following argument:

• Premise: Johnny has 2 apples.
  
  Conclusion: Thus, after going to the store, Johnny has 5 apples.

If you are given the following answer, it would be correct in both an Assumption and Justify question:

(A) Johnny obtained 3 apples for himself at the store.


Now consider this answer:

(B) Johnny obtained 3 apples and 2 oranges for himself at the store.

This answer would be correct for a Justify question (because it gets us to 5 apple number we seek), but it would not be correct for an Assumption question because it has extra stuff about oranges that is not necessary to the argument.


In the first answer, we see how Assumption and Justify can be identical, but in the second we see how they can be very different. Does that help answer your question? Please let me know. Thanks!

[kggaines]

Mr. Killoran,
Makes perfect sense. Your explanation was clear and convincing as always. Thanks so much for your time.