作者Senkousha ( )
看板logic
标题only if, only when
时间Fri May 19 09:29:12 2006
有一个问题困扰我好一阵子了，一段叙述如下：
A person's cholesterol level will decline significantly if that
person increases the number of meals eaten per day, but only if
there is no significant increase in the amount of food eaten.
However, most people who increase the number of meals they eat
each day will eat a lot more food as well.
上面第二行结尾的 only if，被补习班老师解释为 "->"，
强调後面的叙述要放在 "->" 符号的右边，并列了一个式子：
A and B -> C
其中，
A代表"a person increases the number of meals eaten per day"
B代表"a person's cholesterol level will decline significantly"
C代表"there is no significant increase in the amount of food eaten"
用简单的中文代替则是:
A: 多餐
B: 低胆
C: 少量
多餐又低胆 -> 少量
这与多数人的直觉都不符，我个人认为应该套入 1st-order logic 的观念写作：
for all x [多餐(x) and 少量(x) -> 低胆(x)] and
for all y [多餐(y) and 多量(y) -> 高胆(y)]
但是我对自己於 "only if"的解释也感到很奇怪。
想请问大家，问题究竟在哪里？是其中的"will"吗？
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