Truth, consistency, and validity are logical concepts that describe statements and arguments. The concepts are not hard, but a good understanding of them is essential for any serious LSAT student.
In logic, truth means exactly what you think it means. If a statement is true, it accurately reflects reality. If a statement is untrue (false), then it does not accurately reflect reality.
Let’s look at an example of a true statement and a false statement:
- True Statement: The capital city of the United States of America is Washington, D.C.
- False Statement: The capital city of Japan is London.
However, truth has nothing to do with whether or not a set of statements makes sense. Let’s look at example of a nonsensical paragraph that is filled with true statements:
Snow occasionally falls when it is cold outside. Furthermore, pizza is a popular dish in Florida. And if fish didn’t live in water, then they wouldn’t live in the ocean. Given all of this, we can know that South America and Africa are not the same place.
Every statement in the above paragraph is true, but the paragraph still makes no logical sense. The argument is still poor, even though each assertion is true.
Consistency and inconsistency are properties of statements. If you examine any two statements, those statements will be either consistent with each other or inconsistent with each other. If it is possible for both statements to be true at the same time, then the statements are consistent. If it is not possible for the statements to be true at the same time, then the statements are inconsistent.
Another (more common) term for inconsistent statements is “contradictory statements.” If two statement cannot be true at the same time, then those statements contradict each other. But if two statements can be true at the same time, then they are consistent.
Let’s look at a couple examples:
- The sky is blue.
- The sky is not blue.
The above statements are contradictory; if one is true, then the other is not.
Here’s another example:
- Red is a color.
- Purple is a color.
These statements are consistent; they can both be true at the same time.
Consistency comes up on the LSAT with questions that ask you to pick an answer that “could be true” if the stimulus is true. In those cases, the LSAT is asking you to identify the answer that is consistent with the stimulus.
On the other hand, some questions ask you to pick an answer that “CANNOT be true” if the stimulus is true. In those cases, the LSAT is asking you to identify the answer that is inconsistent with the stimulus.
People describe arguments as “valid” in everyday language to describe truth. It’s common to call a statement a “valid point” when we agree with an argument or to call an false statement “invalid.” However, in logic, validity and truth are two separate concepts. It is possible to have a valid argument full of false statements or an invalid argument that only contains true statements.
Logically validity means that an argument’s conclusion can be correctly inferred from the premises. So even if you are not given the conclusion of a valid argument, you can still figure out that conclusion out just from thinking about the premises. In other words, in a valid argument, the conclusion can be logically inferred from the premises.
Let’s look at an example of valid logical argument.
All plants are green. All donuts are plants. Therefore, all donuts are green.
All three statements in the above argument are false, but this argument is still valid. Why? Because logical validity has nothing to do with truth. All that matters is whether or not the conclusion necessarily follows from the premises.
Think about it this way: If we assume that the first two premises are true, then we also have to assume that the conclusion is true. This is what logical validity means; if all the premises are true, then the conclusion must also be true.
Let’s take a look at an argument that is not logically valid (invalid):
Most plants are green. All donuts are plants. Therefore, all donuts are green.
This argument is invalid. Even if we assume that the premises are true, we do not know for sure that conclusion is true. Maybe the conclusion is highly likely, but this is not enough to make the argument valid. In order for the argument to be valid, it would need to be impossible for the conclusion to be false if all the premises are true.
Let’s look at another invalid argument:
If the world was flat, then we wouldn't have seen a round world from space. We went up to space. Therefore, the world is round.
The conclusion of this argument is certainly true, but can we reach it from the premises? No! We are missing a key part of the story; did we see a round world from space or not? Without this piece of information, we do not yet have a logically valid argument. As the argument reads now, it is still possible for the premises to be true while the conclusion is false.
Key Point: Never question truth value on the LSAT. If the LSAT tells you to assume a statement is true, just pretend that it is. The LSAT does not test truth; it tests your ability to recognize consistent statements and logical validity.