以下是引用freegirl在2003-9-22 22:32:00的发言： 注：a -> b, b is required by a we can have more a -> b -> c, then c is required by a but, if a, then b, if b then c, we can't get if a, then c, coz a is a premise of b, not required of b, in other words, in order to establish b, we might need several premises including a.
这里, a -> b 和 if a, then b有区别吗? 如果说if a, then b, if b then c, 那么A是造成B的充分条件, 只要A就足够导致B, 只要B就足够导致C, 为什么A不能导致C呢? 你可以举例证明你的观点吗?
If A, then B, 则A(充分)-->B(必要) “如果A是造成B的充分条件, 只要A就足够导致B”--- 但还有其他的条件也可以导致 B 呀！ 所以我们说 A 是充分的，但不是必要的。如果B 发生了，我们不能说 A 一定发生了，有可能是 D 的发生导致了B 的发生。 举个例子来说：如果你多吃VE，你的皮肤就会好； 如果你的皮肤好，你就会显得比实际年纪年轻。你不能由此推出多吃VE,就会显得比实际年纪年轻。 因为，可能是你天生丽质，不需要VE, 就皮肤好。所以我们推不出一定是多吃VE -〉导致了比实际年纪年轻。 不知道这里例子是不是清楚了。
其实，我还有一点concern: what's the different between if and if only for example: if A then B, 则 A is 充分, B is 必要 B if only A, 则 A is 充分, B is 必要? 还请nn们指正!! 最好也举个例子，加深理解。偶谢过先！
另外: if only: used when you want to say how doing something simple would make it possible to avoid something unpleasant: If only she'd listen to what he's saying, I'm sure they could work it out.
按照CHEN XIANG DONG的解释: IF=IF ONLY, 同样都是充分; ONLY IF则是必要, IF (ONLY) AND ONLY IF是充分必要
oops, I'm sorry I made the mistake, many thanks to jq_jou!
My point is: if a then b(a->b) , if b then c (b->c), therefore, we can get a->c (you're right), but we can't get c->a coz many other reasons can lead to c. 还是拿我举的那个例子：多吃VE, 会显得年轻。但我们不能得到 if looks younger then you certainly take Vitamine E.
In an opposite way： c requires b (c->b), b requires a(b->a), therefore, we can get c->a but we can't get a->c. 再举个例子，要上名牌MBA(b), gmat must over 700(a)； 要进入famous consulting companies(c), 必须先进名牌MBA 修炼(b)。 我们可以得到进入famous consulting companies(c) --> gmat must over 700(a), but we can't get gmat over 700(a) ->进入famous consulting companies(c)
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发表于 2003-9-23 00:24:00
