Two conditional indicators.

victttoriabradley

   What do we do when there are two conditional indicators? For example: 

No student is chosen for Gryffindor unless they exhibit bravery. Therefore, if a student exhibits bravery, they will be sorted to Gryffindor. Lawgic:

A → B

---

B → A

The argument is invalid.

     How come it is not /chosen->/exhibit bravery? indicator “no” and indicator “unless” make negate sufficient condition and negate necessary condition. If we create the contrapositive it makes brave-> chosen which matches the second sentence.

bythenameoftom

I hope I'm understanding your question right. The formula for unless is as follows: take what comes after unless and put it in the necessary condition. Negate what comes before unless and put it in the sufficient condition. For me personally, the contrapositive makes more sense than what the formula gives me at first.

If by two conditional indicators you mean a biconditional, then "if and only if" tells us <---> is what we should diagram.

First sentence: chosen for G -> exhibit bravery, not exhibit bravery -> not chosen for G. The rule is that no student is chosen for G. There is one exception to the rule, being if that student exhibits bravery. They only way to be chosen for G is to exhibit bravery, so if that student has been chosen for G they have exhibited bravery.

Second sentence: exhibits bravery -> chosen for G, not chosen for G -> not exhibit bravery. Pretty simple, the only lawgical rule is that if they exhibit bravery, they will be chosen for G. No exceptions.

Admin correct me if I'm wrong but this is a case of confusing necessary for sufficient.

Far from a master at this, only commenting to reinforce the material for myself. Let me know if you need a study buddy.

victttoriabradley

Ah you are awesome, I think I am seeing some light in the fog. I didn't think of this as an exception. If we don't think about the exceptions, both "no" and "unless" are conditional indicators. So what I was saying is by the conditional indicator rules you can pick one key term and negate it and turn it into the sufficient one (for unless) and pick the other key term negate it and put it in the necessary position respectively (for no).