概率是主观的Probability Is Subjective
Naval：概率在物理宇宙中真的存在吗？还是说，它只是我们无知的产物？如果我掷一颗骰子，我不知道它会落在哪一面，所以我赋予它一个概率。但这是否意味着，宇宙中真的存在某种概率性的、不可知的事物？宇宙是不是在某个地方掷着骰子，还是说它始终是确定性的？
Brett：实际上，所有的概率都是主观的。不确定性和随机性也是主观的。你不知道结果会是什么，所以你才掷骰子。那是因为你个人不知道，而不是因为宇宙深处存在着不确定性。关于量子理论，我们所知道的是：一切在物理上可能发生的事，都会发生。
这就引出了多重宇宙的概念。我们不会去逐一驳斥那些试图理解量子理论却失败的思路，而是要严肃对待量子理论的方程所说的话。鉴于那些实验，我们不得不认为：每一件可能发生的事情，确实都会发生。这意味着宇宙中并不存在固有的不确定性，因为一切可能发生的事都真的会发生。并不是有些事会发生、有些事不会发生——一切都会发生。
你身处其中一个宇宙；在你的宇宙里，当你掷出骰子，它落在"二"上。在物理现实的其他某个地方，它落在"一"上；在另一些地方，它分别落在"三""四""五"和"六"上。
Naval：如果我掷两颗骰子，那么两颗骰子点数之和为二的宇宙，数量要少于点数之和为七的宇宙，因为后者可以是三加四、五加二，等等。所以宇宙的数量仍然与我们计算出的概率相对应。
Brett：是的。这就引出了戴维·多伊奇所说的"决策论式"的理解概率的方式——在量子理论的框架内理解概率。所谓决策论式，是指你假定宇宙在分裂时是按照某种比例进行的。所以如果你掷两颗不同的骰子，宇宙就会按照一定的测度来划分自己。测度是一种谈论无穷的方式。
Naval: Does probability actually exist in the physical universe, or is it a function of our ignorance? If I’m rolling a die, I don’t know which way it’s going to land; so therefore I put in a probability. But does that mean there’s an actual probabilistic unknowable thing in the universe? Is the universe rolling a die somewhere, or is it always deterministic?
Brett: All probability is actually subjective. Uncertainty and randomness are subjective. You don’t know what the outcome’s going to be, so you roll a die. That’s because you individually do not know; it’s not because there is uncertainty there deeply in the universe. What we know about quantum theory is that all physically possible things occur.
This leads to the concept of the multiverse. Rather than refute all of the failed ways of trying to understand quantum theory, we’re going to take seriously what the equations of quantum theory say. What we’re compelled to think about quantum theory, given the experiments, is that every single possible thing that can happen does happen. This means that there is no inherent uncertainty in the universe because everything that can happen actually will happen. It’s not like some things will happen and some things won’t happen. Everything happens.
You occupy a single universe, and in that universe, when you roll the die, it comes up a two. Somewhere else in physical reality, it comes up a one, somewhere else a three, a four, a five, and a six.
Naval: If I’m rolling two dice, then the universes in which they sum up to two is less than the number of universes in which we roll a seven, because that can be a three and a four, a five and a two, and so on. So the number of universes still does correspond to what we calculate as the probability.
Brett: Yes. This leads to what Deutsch calls their decision-theoretic way of understanding probability within quantum theory. Decision-theoretic means you assume there’s proportionality between the universes’ way of splitting things up. So if you’re rolling two different dice, then the universes proportion themselves into measures. A measure is a way of talking about infinities.