# The author uses the word "immediacy" (line 39) most likely in order to express

Alexis-Greene on December 27, 2019

How do you know if a question is valid or invalid?

I am confused on how you are determining if a question is valid or invalid and then from there how you determine if they just negated or just reversed the variables. How do you determine the flawed reasoning they used?

AndreaK on January 31, 2020

Hi @Alexis-Greene,

When I try to view your question on my end, because it's not a single LSAT problem, the entire lesson module pops up. Would you mind pointing out where in the videos or examples you are specifically referring to so I can better answer this for you? In the mean time, I'll break down valid and invalid for you and maybe this will help clarify your questions.

As you know, the contrapositive is valid. We reach the contrapositive by BOTH reversing and negating.

Let's use a real world example for clarity, one that everyone can easily picture in their minds. I know that sometimes the unintuitive nature of using As and Bs can make a simple concept seem tricker than it is, so I like to opt for everyday real world illustrations when I can.

"All cats are mammals."

Cat - > mammal

/mammal/ - > /cat/

It's true that if you're not a mammal, you certainly aren't a cat. That's because the mammal kingdom completely encompasses the cat species, so that's the necessary condition.

When you reverse AND negate, you have created a formula that is logically valid. However, arguments that only reverse OR negate (but don't do both) make a critical mistake.

All cats are mammals.

Cat - - > mammal

Mistaken reversal:
Mammal - -> cat

If you're a mammal, then you're a cat. But, that's not necessarily true! Because you could be a dog, or a squirrel, or, in our case, a human :). So, you can be a mammal without being a cat. That's why this is no good.

Mistaken Negation:

/cat/ - > /mammal/

If you're not a cat, then you're not a mammal. Again, that's bad logic! Like above, this doesn't account for the fact that you could be a dog, a squirrel, or a human, all of which are mammals.

The above examples show why only reversing or only negating is invalid. And, they show why doing both is key! So hopefully this helps you see the reasoning behind diagramming a little more clearly.