I am probably 3 years too late but at 9:08 you say that we cannot conclude B from any of the premises so we put B as a subproof. What prevents us from being to conclude B and on the next line G from B -> G in the main proof using conditional elimination?
There simply is no rule that allows you to conclude B from the premises. That is, if you wrote B on line 4, say, without it being an assumption, there would be no way to justify it with any rule. So, there would be no way to get G by conditional elimination, since for that you need B in addition to premise 1.
![[Logic] Proofs and Rules #1](http://i.ytimg.com/vi/m2j0TX-e8NY/mqdefault.jpg)
![[Logic] Proofs and Rules #1](/img/tr.png)
Youre the best neil. I love you.🦍🦧🎺🔕🎼🎶🎧
This is fantastic, thank you so much for this!
You're welcome, and thanks for the kind words!
I am probably 3 years too late but at 9:08 you say that we cannot conclude B from any of the premises so we put B as a subproof. What prevents us from being to conclude B and on the next line G from B -> G in the main proof using conditional elimination?
There simply is no rule that allows you to conclude B from the premises. That is, if you wrote B on line 4, say, without it being an assumption, there would be no way to justify it with any rule. So, there would be no way to get G by conditional elimination, since for that you need B in addition to premise 1.