Logical inferences and logical scope are related terms, and both are important for the LSAT. Let’s take a look at both concepts.
Logical inferences are pieces of information derived from statements that we either know to be true or assume to be true for the purpose of argument. In other words, inferences are statements we can deduce from other statements. If those statements are true, then the inference must also be true.
Let’s look at a couple statements for an example.
- I love all books.
- Everybody Poops is a book.
If both statements above are true, then we can also say with confidence: “I love Everybody Poops.” The statement “I love Everybody Poops” is not directly said anywhere in the two statements above, but we can infer “I love Everybody Poops” from the above statements just the same.
Note though that the inference is not just likely; it must be true. You will get yourself in a lot of trouble on the LSAT if you make logical inferences based on what is likely. Logical inferences are what you know to be true if the premises are also true.
Some inferences are difficult to understand, but others Inferences are painfully obvious. For instance, if we know that “Sam is in a green room,” then we also know “Sam is in a room.” The second statement can be inferred from the first. Here’s another example: If we know “Cindy owns at least thirty books about pottery” then we also know, “Cindy owns at least twenty books about pottery.” The first claim cannot be true unless the second claim is true. We can therefore infer the second claim from the first claim. Pointing these inferences out may seem silly, but it is important to understand that these statements can in fact be logically inferred.
Note: Logical inferences (used often in the logical reasoning sections of the LSAT) are different from reading comprehension inferences.
An argument’s scope is the range of that argument’s topic. Think of it as the full range of the argument’s claims. The scope consists of all the information that can be drawn from the argument; it is a range of inferences. Thus, saying that an assertion is “out of scope” means that the statement cannot properly be drawn from the information provided in the argument.
Let’s look at an example.
Brenda is cheating on her boyfriend. I saw her at the club last night kissing a brown-haired guy, and her boyfriend is completely bald.
So what is this argument talking about? The topic is Brenda cheating on her boyfriend. The inference that Brenda has a boyfriend is therefore within the scope of the argument; this assertion is a part of the argument. Saying that Brenda is a bad person, however, is not within the scope of the argument. The argument never states whether cheating is good or bad; it only says that Brenda did it. We cannot get to Brenda being a bad person unless we widen the scope of the argument by adding new information (like the judgement that cheating is bad).
Let’s look at a few more potential inferences and determine whether or not they are within the scope of the above argument.
- Brenda was at the club last night.
Within the scope of the argument! If Brenda was actually seen at the club, then she must have been there. We can make this inference without adding any extra information.
- Brenda knew she was cheating.
Out of scope. We have no idea whether or not Brenda was aware of her actions. Nothing in the argument requires this statement to be true.
- Brenda likes bald men.
Out of scope. We have been given no reason to believe that Brenda likes bald men. Her boyfriend is bald, but do we know she likes her boyfriend? Furthermore, “men” is plural; perhaps she only likes one bald man. This statement is not a legitimate inference.
Here’s the important take-away: A good inference must be within the argument’s scope. If an assertion draws on any information that does not originate from the argument or from another legitimate inference, then it is not a legitimate inference.