好的解释难以变动Good Explanations Are Hard to Vary
Naval： 布雷特，你会不会认为科学理论是"好的解释"的一个子集？
Brett： 是的。它们是那种可检验的好的解释。可证伪的理论其实比比皆是，这并不能说明你听到的解释质量如何。
《真实世界的脉络》(The Fabric of Reality)中举的例子是"吃草治感冒"。如果有人说"吃 1 公斤草就能治好你的感冒"，那他们确实有一套可检验的理论。问题是没人应该去检验它。为什么？因为他们没有解释草为什么能治好感冒的机制。而如果你真的吃了 1 公斤草却没治好，他们又会改口说："也许 1.1 公斤就行。"
Naval： 对。或者你需要换一种草。
Brett： 它总是可检验的，但你没有任何进展。
Naval： 好的解释的第二个特征是"难以随意改变"。它必须非常精确，而且这种精确必须有充分的理由。
《无穷的开始》(The Beginning of Infinity)中引用的著名例子，是关于四季成因的解释。古希腊的解释是，四季由春之女神珀耳塞福涅（Persephone）以及她何时能离开冥界所决定。那是一整套涉及诸神女神的理论。它不仅难以检验，而且极易随意改动。珀耳塞福涅可以是奈姬（Nike），冥界也可以是朱庇特或宙斯。这个解释很容易被改动，而预测却丝毫不变。
而如果你看地轴倾角理论——它认为地球相对太阳倾斜 23 度，因此我们可以预期太阳冬天从这边升起、夏天从那边升起——这个理论的事实很难随意改动。它做出的是高风险且狭窄的预测。地轴倾角理论可以预测不同纬度夏季和冬季的精确长度，而且你可以精确地检验这一点。
除了是一门可检验、可证伪的创造性理论之外，这个理论的各个组成部分还应该难以改动——除非实质上推翻整个理论。而且你当然不想在事后去改动它——就像你那个吃草的例子："哦，是 1 公斤？不，现在是 1.1，现在是 1.2。"
最后，它做出的预测应该是狭窄、精确且高风险的。比如，我记得在相对论中，是爱丁顿做了那个实验，证明日食时星光会发生弯曲。那是爱因斯坦在相对论中做出的预测，最终被证实为真。那是一个高风险、且历经很长时间才得到证实的预测。
Naval: Brett, would you say that a scientific theory is a subset of a good explanation?
Brett: Yes. They’re the testable kinds of good explanations. Falsifiable theories are actually a dime a dozen. This doesn’t tell you anything about the quality of the explanation you’re being given.
The example that’s used in The Fabric of Reality is the grass cure for the common cold. If someone says, “If you eat 1 kg of grass, it will cure your common cold,” then they have a testable theory. The problem is that no one should test it. Why? Because they haven’t explained the mechanism that would enable grass to cure the common cold. And if you do eat 1 kg of grass and it doesn’t cure your cold, they can turn around and say, “1.1 kg might do it.”
Naval: Right. Or you need a different kind of grass.
Brett: It’s always testable, but you’re not making any progress.
Naval: The second piece of a good explanation is that it’s hard to vary. It has to be very precise, and there has to be a good reason for the precision.
The famous example used in The Beginning of Infinity is the explanation for why we have seasons. There’s the old Greek explanation that it’s driven by Persephone, the goddess of spring, and when she can leave Hades. There was this whole theory involving gods and goddesses. Not only was that not easily testable, it was very easy to vary. Persephone could have been Nike, and Hades could have been Jupiter or Zeus. It’s very easy to vary that explanation without the predictions changing.
Whereas, if you look at the axis tilt theory—which says Earth is angled at 23 degrees relative to the sun and therefore we’d expect the sun to rise here in the winter and over there in the summer—the facts of that are very hard to vary. It makes risky and narrow predictions. The axis tilt theory can predict the exact length of summer and winter at different latitudes, and you can test that precisely.
Beyond it being a creative theory that is testable and falsifiable, it should be hard to vary the pieces of that theory without essentially destroying the theory. And you certainly don’t want to vary it after the fact—like in your grass example, “Oh, it was 1 kg? No, now it’s 1.1, now it’s 1.2.”
Finally, the predictions that it makes should be narrow and precise, and they should be risky. For example, I believe in relativity it was Eddington who did the experiment and showed that starlight gets bent around an eclipse. And that was a prediction that Einstein had made in relativity, which turned out to be true. That was a risky prediction that took a long time to confirm.