17. Whatis formal validity_ 17.什麼是形式有效性_ Segment 17, what isformal validity? 什麼是形式有效性？ In the previoussegment, we saw that an argument's valid if, and only if it's impossible forits premises to be true when its conclusion is false. In this segment, I willdiscuss a special way of being valid called formal validity. What's that? Well,an argument is formally valid when it is valid by virtue of its form. 在上一節中，我們看到一個論點是有效的，當，並且僅當它的前提不可能是真的，當它的結論是錯誤的。在這一部分中，我將討論一種特殊的有效性方式，即形式有效性。那是什麼？好吧，一個論點在形式上是有效的，那麼它在形式上是有效的。  Great. Or whatdoes that mean? Let's start with a simple exam. Consider this argument. Alex isa woman and Bobby is a man. So Alex is a woman, pretty simple. I've told you.How can we isolate the form of this argument? Just replace each sentence with avariable. The first sentence is Alex is a woman. 偉大的。或者那是什麼意思？...