What is a necessary versus a sufficient assumption?
- Louis Zatzman
- 2 days ago
- 5 min read
The best way of understanding question types on the Law School Admissions Test (LSAT) is by understanding what the correct answer for each given question type actually accomplishes. In other words, what makes a right answer right?
One of the most confusing question types on the LSAT is the assumption question. An assumption on the LSAT is synonymous with a flaw, meaning the passage given to you for an assumption question is by definition flawed, or missing proof. It is a dynamic question, meaning the correct answer changes the passage in some way. Not the truth of any element in the passage, but the quality of the proof within it. And to complicate things further, there are two types of assumption questions: sufficient and necessary.
On one hand, both sufficient and necessary assumptions are types of sufficient and necessary conditions. So to understand the question types, you have to understand what those conditions actually constitute. You can do that here!
But ultimately, a sufficient assumption is going to fully prove the conclusion offered within the passage. It is sufficient, which means it is causative, and it is causing the conclusion to be proven. A necessary assumption is not going to prove the conclusion, but it is required to prove it. (Again, understanding your formal logic helps a lot here.) In other words, if the necessary assumption is wrong, the conclusion will also be wrong. So you can think of sufficient and necessary assumptions as reaching opposing destinations within their individual passages; the sufficient is proving the conclusion, the necessary is in many ways disproving it (if it is missing).
Example Assumption Passages:
Let's give a few examples with actual logic:
Passage: John is taller than Bill because John is 6-foot-1.
Answer choices:
A) Bill is 5-foot-4.
B) Bill is not 6-foot-2.
Which do you think is sufficient, in that it fully proves the conclusion, and which do you think is necessary, in that it is required to prove the conclusion?
A) goes all the way to proving the conclusion, so it is a sufficient assumption. It is not required, though! Bill being 5-foot-2, for example, would also prove the conclusion. So this would be a sufficient assumption.
B) does not prove the conclusion, as Bill could still be 6-foot-4, but it is required. Because if Bill is 6-foot-2, the argument would be wrong. So this would be a necessary assumption.
Here’s another example passage, this time with a series of possible answer choices. The idea here is not to identify one answer choice as correct, but to prove that multiple answers can accomplish the same goal. There’s no such thing as one assumption.
First the passage:
Bill or James could have committed fraud in our company. We know that fraud was committed, and it wasn't by an external investor. Only one person committed the crime. Bill claims he's innocent, which is supported by all incontrovertible evidence. So James must have committed the fraud.
(By the way, if you're having trouble identifying the conclusion, which is crucial to solving assumption questions, you can learn about the process here.)
Now here’s a series of answer choices, all of which are either sufficient assumptions (they prove the conclusion), necessary assumptions (they are required to prove the conclusion), or neither. Going through these should help you understand what an assumption actually accomplishes.
James is not an internal investor.
Some of the people being considered as culprits aren’t considered to be likely candidates.
James is not an external investor.
No internal investor other than Bill or James committed the fraud.
Sally didn't commit the fraud.
The evidence exonerating Bill is correct.
Bill and James are the only two people who could have committed the fraud.
Bill and James are both external investors.
Sarah did commit the fraud.
No external non-investor committed the fraud.
James is an internal investor.
There’s some evidence that James had motivation to commit the fraud and the opportunity to do so.
Think about each, and try to answer them on your own before you look at the answers. Remember: The best way of understanding question types on the LSAT is by understanding what the correct answer for each given question type actually accomplishes. In other words, what makes a right answer right?
Not an assumption: This doesn’t prove James did commit the fraud, nor does its negation prove he did not.
Not an assumption: Too weak to be an assumption.
Necessary assumption: This is required in order to prove James having committed the fraud because we know external investors are innocent.
Necessary assumption: This is required to prove the fraud, because if someone (whether an internal investor or otherwise) else committed the fraud, which only one person committed, then James certainly did not.
Necessary assumption: This is required to prove the fraud, because if someone else committed the fraud, which only one person committed, then James certainly did not.
Not an assumption: This is not even new information; we already knew that the evidence was correct, given by the word “incontrovertible.” Assumptions have to be dynamic, so this cannot be an assumption.
Sufficient assumption: This fully proves that James committed the crime when it is added into the passage and combined with the other premises.
Not an assumption: Whether Bill and James are or are not external investors, internal investors, whatever, none of it proves or is required to prove the conclusion.
Not an assumption: This actually would make the argument flat-out wrong, so it is certainly not an assumption that either proves or is required to prove the conclusion.
Necessary assumption: This is required to prove the fraud, because if someone (whether an investor or otherwise) else committed the fraud, which only one person committed, then James certainly did not.
Not an assumption: Whether Bill and James are or are not external investors, internal investors, whatever, none of it proves or is required to prove the conclusion. Perhaps James not being an external investor, who did not commit the crime, makes it more likely he did commit it. But assumptions don’t deal with likelihoods.
Not an assumption: This might make it more likely that James committed the crime, but it doesn’t prove it, nor is it required to prove it. Assumptions are not strengthening arguments, nor do they deal in likelihood. In other words, this doesn’t combine with the premises offered, but instead tries to replace them.
These examples should show the differences between sufficient and necessary assumptions. Ultimately, every question is defined by what the answer accomplishes. And understanding the difference for these question types cuts into the heart of much of the other material required to succeed on the LSAT.
