我突然觉得这和所谓可能世界有关。对它久仰大名，但一直懒得去细读。我的本能感觉是可能世界是个不可能成功的理论，因为说不清楚到底哪些属于可能，哪些属于不可能。（或许除了apriori都可能？）

个人感觉，physcalism的真正争议只有一小部分是关于心物问题，起码有一大半是关于可能世界和语言指代的。这个现象可谓意料之外，情理之中。

There are two cencepts I find very similar.
(1)【a posterori necessary】 Consider "water is H20 ". Is this a necessary fact? Well, there is a possible world which looks very like ours: there are oceans and lakes, people need to drink the liquid in them to survive. The only difference is that the chemistrists at that world finally find that water is H30 rather than H20. It seems intuitively correct to say that "water is H20 " is false at this world.
However, according to some people, this intuition is incorrect. The world seems to contain water but actually it does not. So we can not conclude that "water is H2O " is a false statement from the fact that "the liquid in the oceans and lakes is H30". Because the liquid in the oceans and lakes is just not water. So they believe that there is no possible world such that "water is H2O" is false. In other words, it is a necessary fact.
(2)【nomological necessary】 Intuitively speaking, it is something like "there is no nomologically possible world where an object travels faster than light speed".
My question is: are these two concepts talking about the same issue,or are they equivalent? My intuition is that this is the case. Actually, I think in some sense (1) is based on (2), is an application of (2), but is essentially nothing more than (2).
Thanks for reading,

After some more reading, my new understanding is completeluy reverse. It seems that (2) is an apllication of (1). This is not exactly what I think. Actually, I think, (2) is a misunderstanding of (1).
I mean, (1) and (2) are completely irrelavant. Whoever thinks that (1) implies (2) is just misunderstanding (1). After all, it is really really absurd to believe that (1) actually implies (2), a game of language actually implies a mataphysic result.
I will use a concrete example to illustrate my understanding of (1). The necessary truth "Hesperus is Phosphorus" does not mean
(3) There is no such possible world: there is a heavenly body people call Hesperus, and another heavenly body people call Phosphorus. People later find that Hesperus is not Phosphorus.
Rather, it means
(4) There is no such possible world: the heavenly body which we 【actually】 call Hesperus exists, and the heavenly body which we 【actually】 call Phosphorus also exists. But the first body is not the second body,
which is equivalent to:
(5) There is no such possible world: Venus exists in this world. But Venus is not Venus.
I think this is nothing but a game of language. Thanks for reading.

随着进一步的阅读，我的理解也陷入了进一步的混乱。不管那么多，还是记录一下此刻的理解。
这次考虑 “任何可能世界，不能违背万有引力定律”。表面来看，很容易想象某个世界里万有引力定理不成立。比如说，那个世界里 （而不是平方）。但我们可以说，对这个世界的正确描述并不是“那里万有引力定理不成立”，而是“那里有一种新的物体，不妨称为类物质；类物质没有质量，但有一种不妨称为‘类质量’的类似性质，‘类质量’遵守着‘类万有引力定律’”。
就像在水的例子中，那个可能世界里大海和河流里并没有水，有的是一种全新的物质一样；这里说的可能世界里也并没有任何现实世界里存在的物质，有的是一种全新的“类物质”。而就像在现实世界里人们渐渐发现了暗物质一样，人们也完全有可能会在现实世界里发现类物质。一言以蔽之，不同的物质遵守不同的物理规则。而现实世界的物质恰好遵守着万有引力定律，所以如果某可能世界里也存在着现实世界的这些物质，它就必须遵守万有引力定律，否则它就绝不是现实世界里的物质。
你可以说这是在搞毫无意义的语言游戏，但我觉得更合适的说法是，这是一种看待世界（准确地说，看待可能世界体系）的角度。总之，在这种角度下，人类一旦发现了某种自然律，这自然律就成为了形而上学必然，同时也是后天必然。所以在这种角度下，自然律必然=后天必然=形而上学必然，即最必然的必然。
估计过几天理解还会变。这里只是记录一下。

我现在真正理解zombie帖子里说的是啥问题了，这似乎是epiphenimenalism面对的最大的问题（不讨论自由意志）