Negating Statements By Quantity or "Not"

extramedium

I've run into this problem a few times on NA questions, where I can't decide whether to negate by adding a not or negating the quantity. Here's an example of an answer choice from an NA question:

There are some illnesses that experienced physicians can diagnose accurately from physical examination alone.

In this instance, would you negate some to none or place a not after can to negate? Or both?

Not Ralph Nader

I usually negate some to none but it depends on what is inside stimulus and what the particular answer choice that I am trying to negate says. However if you are looking for a more general way to negate you can negate an answer choice by applying the phrase, "It is not the case that" to the answer choice as a whole.

Let's apply "It is not the case" to this answer choice and see which one of the options represent the meaning of It is not the case that [There are some illnesses that experienced physicians can diagnose accurately from physical examination alone.] more accurately.

1- Negating some to none/no: There are no illnesses that experienced physicians can diagnose accurately from physical examination alone.

2- Placing a not after can to negate: There are some illnesses that experienced physicians cannot diagnose accurately from physical examination alone.

Both of these statements are factually correct in a world in which, we apply "It is not the case" to the answer choice as a whole. But as you can see negating some to none is more accurately represent the situation because if there are no illnesses that experienced physicians can diagnose accurately from physical examination alone; it would be definitely true that there are some illnesses that experienced physicians cannot diagnose accurately from physical examination alone. However based on the second statement [There are some illnesses that experienced physicians cannot diagnose accurately from physical examination alone.] we cannot conclude we the first statement [There are no illnesses that experienced physicians can diagnose accurately from physical examination alone.] must be true.

nessa.k13.0

Yup! I totally agree with @"Not Ralph Nader"

extramedium

Does this apply across the board? So that if there is a quantity indicator in a statement, say for an NA question, I should always negate that rather than the necessary indicator? Words like can, have, must etc.

Thanks for the response @"Not Ralph Nader"

Not Ralph Nader

@"Aaron Frank" it is hard to say something applies across the board in LSAT. It depends on the context. There are some general rules of thumb that most of the time works. For example, negation by changing the quantity indicator, like changing "Some" to "None", "Most" to "Some" or "All to Most/Some". But I would suggest to use "It is not the case" phrase and apply it to the answer choice whenever you are in doubt between negating quantity indicator or negating the verbs like can, have,etc.

For instance, the answer choice you mentioned (There are some illnesses that experienced physicians can diagnose accurately from physical examination alone.) "that" is a modifier indicator so the emphasis shifts toward the quantity indicator in this case because whatever comes after "that" describes "illnesses" therefore, you can negate this answer choice by negating the quantity indicator "some". LSAT writers can and probably will design a answer choice that defies those rules of thumb I just described above.

extramedium

Alright this makes sense. The "it is not the case" method seems relatively foolproof if I come up short. But tell me if this seems accurate. I'm pretty sure that, with the exception of statements containing all, any, every, statements including quantifiers should be negated by quantifier.

The issue with the 'all' indicators is that 'not' is always included in the negation. (All dogs are cute, some dogs are not cute)

Negating some, most and the others does not involve placing a 'not' in the sentence.