命題

參考：http://d.hiphotos.baidu.com/baike/c0%3Dbaike80%2C5%2C5%2C80%2C26/sign=17ee5699820a19d8df0e8c575293e9ee/29381f30e924b899725076106c061d950b7b02087bf4d696.jpg

 

對於兩個命題，若是一個命題的條件核結論分別是另外一個命題的結論和條件，那麼這兩個命題叫作互逆命題，其中一個命題叫作原命題，另外一個命題叫作原命題的逆命題；

若是兩個命題其中一個命題的條件和結論分別是另外一個命題的條件和結論的否認，則這兩個命題稱互爲否命題。

We can form some new conditional statements starting with a conditional statement p->q. In particular, there are three related conditional statements that occur so often that they have special names. The proposition q->p is called the converse of p->q. The contrapositive of p->q is the proposition ¬q->¬p. The proposition ¬p->¬q is called the inverse of p->q.