Only a scientist could understand all that this pursuit meant.

关于这个句子，我跟二位老师的看法不同。

我认为，这个句子不能说一定是“虚拟语气”，但可以说，它完全有虚拟语气的可能性。至少50% 以上。

在含有if 条件句的复合句中，一般通过主、从句的谓语动词形式，即可判断出是否用了“虚拟语气”。但很多情况下，虚拟的条件并不是直接通过条件句来表示，而是用一些词、短语或其他连词，甚至通过上下文的暗示，这就是所谓的含蓄条件。有时候还有更含蓄的，句子主语本身就是一种含蓄条件。例如：

1. Any thinking person would not accept such a condition.

任何一个有思想的人都不会接受这个条件。

等于说：If he was a thinking person, he would not accept such a condition.

2. A wiser man would not have done such a foolish thing.

聪明一点的人是不会干出这等蠢事的。

等于说：If he had been a wiser man, he would not have done such a foolish thing.

网友的句子，跟上述情况相同，Only a scientist 也存在含蓄条件。

Only a scientist could understand all that this pursuit meant.

这句话蕴含着这层意思：

An ordinary person can’t understand all that this pursuit meant. He could understand all that this pursuit meant only if he were a scientist.

【翻译】一个普通人无法理解这种追求的全部含义。只有一名科学家，他才能懂得这种追求的全部意义。

我再举几种其他情况的含蓄条件。

（1）动词不定式表示含蓄条件：

You would be a fool to refuse his offer.

你要是拒绝他的提议，就太傻了。

等于说：If you should refuse his offer, you would be a fool.

（2）上下文表示含蓄条件：

She would have come to see you，but she was so busy that day.

她本来会来看你的，可是那天她实在太忙了。

等于说：If she had not been so busy that day，she would have come to see you.

Such mistakes could have been avoided.

（如果我们再仔细一些）这样的错误本来是能够避免的。

等于说：If we had been more careful,such mistakes could have been avoided.