But the youthful conviction that the world is there any chance to get together again, for novalidreason a person does not appear in another person's life.
但秋水坚信这世上没有偶然的相逢，一个人不会无端地出现在另一个人的生命里。
2
And the reason for that because an argument is valid if and only if there are no possible situation in which all its premises are true and its conclusion false.
原因是因为论点有效，如果且仅如果没有可能情况，所有的前提都为真结论为假。
3
If the efficient market hypothesis were valid, there would be an a priori reason for imposing no constraints.