- What does "since" mean? It's a premise indicator.
- And "because"? Also a premise indicator.
- What does "needs" mean when you want to translate a sentence to conditional logic? The thing that's needed = necessary condition.
- What about "if"? Sufficient condition.
- And "only"? Necessary condition.
Now there's nothing wrong with following rules. Rules about how to translate various words on the LSAT exist for a reason - they've proven to be effective ways to get students to interpret statements accurately and to avoid common mistakes. Knowing that "only if" is very different from "if" is critical to doing well on the test, and if it takes memorizing rules to consistently distinguish between these words, then memorize away.
But of course, there's a limit to your performance if you rely mainly on mechanical application of LSAT rules in logical reasoning. There are innumerable ways to phrase ideas in the English language, and the same word can have various meanings depending on context.
Let's go through the bulleted list of examples one by one.
The subway must be late, since there's a huge crowd on the platform.
In that sentence, "since" is indeed a premise indicator - it's providing readers with the reason that the author thinks the subway must be late.
But what about here:
The mayor has been reluctant to speak to the media since the launch of the FBI investigation into her campaign financing.
"Since" in this example is simply a statement about timing: from the launch of the investigation until the present time, the mayor has been reluctant to speak to the media. Treating "since" as a premise indicator would be a complete misreading. This sentence isn't presenting a premise/conclusion structure - it's simply stating a fact about the mayor's reluctance to speak.
Consumption of vegetables is morally wrong because vegetables feel pain.
"Because" is introducing the reason that the author believes consuming vegetables is morally wrong. So "because" does indeed introduce a premise.
But let's examine this:
The children looked forward to the blizzard because they believed it would lead to school being canceled.
Here, "because" is telling us why the children looked forward to the blizzard, but this sentence isn't presenting an argument. The author of the sentence isn't trying to convince us that the children looked forward to the blizzard. Rather, the author is stating as a fact that the children did look forward to it and is telling us another fact - the reason they looked forward to it is that they thought school would be canceled. Treating this sentence as an argument with a premise/conclusion structure wouldn't be sound.
Nancy needs to find water to survive.
In this example, "needs" is describing a necessary condition. If she is to survive, it is necessary that she find water. If she doesn't find water, then she won't survive.
But, you know it can get more complicated:
All that the investigator needs to identify the intruder is a single strand of hair.
Is this saying that a single strand of hair is a necessary condition for identifying the intruder? That if the investigator doesn't get a single strand of hair, she can't identify the intruder? Not at all. It's actually saying that a single strand of hair is sufficient for the investigator to identify the intruder. But it's not ruling out that there could be other ways to identify the intruder - perhaps fingerprints, blood samples, saliva, or other sources of DNA could work. English is weird, isn't it? The phrase "All that is needed" isn't actually saying that something is necessary.
If and only
If you are on academic probation, you are not eligible for the internship.
Only people not on academic probation are eligible for the internship.
In these examples, which both mean the same thing, "if" introduces a sufficient condition, and "only" introduces a necessary condition. So these words have their traditional LSAT-rule meaning. Being on academic probation is sufficient to be ineligible, and similarly, *not* being on academic probation is necessary in order to be eligible.
But check this out, from an actual LSAT exam:
Books of local historical significance can only be removed from circulation if they have not been checked out for over three years.
Does "If" introduce a sufficient condition here? Does this mean "If they have not been checked out for over three years -> books of local historical significance can only be removed"? Not quite. This sentence is actually saying that not having been checked out for over three years is necessary in order for the books to be removed. You can rephrase the sentence by putting "only" and "if" together to become the familiar phrase "only if", which introduces a necessary condition: "Books of local historical significance can be removed from circulation only if they have not been checked out for over three years."
So what are we to make of these examples that may not fit the standard rules that you've memorized? Some of them probably intuitively made sense to you, right? I'm certain that you've used "since" to refer to timing and that you understand "because" doesn't always introduce a premise in support of a conclusion. And the phrase "all that is needed" probably struck you as a little bit strange - did you suspect that it wasn't quite as straightforward as "needed" = necessary?
Ultimately elite LSAT performance requires that we develop this kind of sensitivity to language. Rules have their place, but we should be careful when their mindless application results in strange interpretations. Sometimes, that's because our own interpretation is in fact incorrect and we should follow the rule. (This is often the case when students read the word "unless" or "except".) But other times, our language "Spidey-sense" is right, and we should put our trust in ourselves as readers rather than in robotic LSAT rules.
Future LSAT Reading Comp Passages
This section of the post collects interesting online articles that you might see edited down to an actual LSAT RC passage in the distant future. Read these recommendations every week and I can guarantee that your RC score will improve or you'll learn something interesting about the world, or both.
🤖 Artificial Intelligence Will Do What We Ask. That’s a Problem. | Quanta Magazine
⛪ The Yale Law Journal - Forum: Religious Exemptions and the Family
🐖 You Can Save More Animals by Donating $100 Than Going Vegan (nautil.us)
🦻 On the art and science of making buildings sound natural | Aeon Essays