SDCAR2010【逻辑入门】(十四)Formal Logic (2)
Unless
The word unless can also create an if-then relationship. But it can be the most confusing and counterintuitive “logic” word. Consider this example:
Your CR score will not be high unless you study formal logic.
There are two ways to translate this statement into if-then clauses:
1) If your CR score is high, then you have studied formal logic.
2) If you did not study formal logic, then your CR score will not be high.
The second method is the foolproof way, which replaces unless with if not. The preferred way is the first method, which negate the clause before unless and cap it with if, and replace unless with then. Basically the clause after unless is the necessary condition which must happen for the negated form of the other clause. Back to the example we have here, “study formal logic” is a necessary step for one to get high score in CR. Without “studying formal logic”, one would not be able to score a high mark in CR. But “studying formal logic” alone might not be sufficient to help you score high in CR.
When the unless-clause comes at the beginning of the sentence, everything between the word unless and the comma is the unless-clause.
Either
The word either can also create an if-then relationship. Consider this example:
Either Peking University or Tshinghua University is on the list of my dream schools.
Given this rule, if PKU is not on my list, then THU is on my list because one of them must be. Further, the rule does not exclude the possibility that both schools are on my list. Therefore, the correct way to say the same thing using if-then clauses is:
