Identifying correct sufficient/necessary terms

@hales, Great question. The truth is that it doesn't really matter how you diagram it as long as you understand what the symbols you're using are referring to so you don't get confused. I recommend using the shortest possible symbols/words/phrases for you to capture what's going on. This way, you're being efficient, but you're also covering everything. Instead of writing "distinguish," you could even just write "D" to connote the phrase you're identifying as the necessary premise. Does that make sense? Let us know if you have any other questions!

I have a question, when to use contrapositive/positive argument? I see that sometimes he does and sometimes he does not? Why makes it invalid to conclude sufficient condition? The don't just reverse/negate part!

The contrapositive is the logically equivalent statement to the original one. Example original statement: A - >B Contrapositive: /B - >/A These two statements mean the exact same thing. You can "use" the contrapositive when it makes it easier to see why something could be concluded. Regarding concluding the sufficient condition, that's never something that we're able to do. We can either conclude the necessary condition or the FAILURE of the sufficient condition. A - >B A is the sufficient condition and B is the necessary condition. We can't conclude A; rather, A is given to us. /B - >/A /B is the sufficient condition and /A is the necessary condition. /A is also the FAILURE of the sufficient condition of the original statement. For more info on this stuff, I recommend reviewing our video lessons on S&N, as there are tons of drills there that will allow you to better familiarize yourself with the material. Hope this helps. Let us know if you have any more questions!