二律背反：康德的悖论 Antinomies: Kant's Paradox

二律背反是康德提出来的。就是说，两个命题，单独看都可成立，放在一起，却相互矛盾。

他列举了四组二律背反：

①正题：世界在时间上有开端，在空间上有限；反题：世界在时间上和空间上无限。

②正题：世界上的一切都是由单一的东西构成的；反题：没有单一的东西，一切都是复合的。

③正题：世界上有出于自由的原因；反题：没有自由，一切都是依自然法则。

④正题：在世界原因的系列里有某种必然的存在体；反题：里边没有必然的东西，在这个系列里，一切都是偶然的。

看起来，世界的空间和时间是有开始和会终止的。每件事情都有始终，不是吗？可是，世界又应当有无限空间和时间，不然的话，空间极限外和时间开始前又是什么呢？

康德认为，他提出的本体和现象概念解答了这个悖论。本体是纯粹概念，没知觉的世界。现象是经验的世界。

空间和时间都不是事物的本体，也不是我们经验的对象。它们是我们经验中所体验到的形式，是脑海呈现给我们的世界特色。谈空间和时间的开始和有限性，就是错误的把它们当做一种物体。

相同的，我们可以被引向认为这个世界的事物的发生是偶然性的。也可以认定是必然性的，因为世上的事都先有前因才有后果的。康德认为，自然规律的确决定了事物的发生，可是那只限于自然现象；在本体世界，自由意识是完全可能的，而伦理道德就是其中的例子。

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Kant, the great philosopher, pointed out that philosophy, or the use of reason, can seem to lead to contradictions. He gave examples of four antinomies:

The first antinomy has as thesis that the world has a beginning in time and is limited in space, and as antithesis that it has no beginning and no limits.

The second proves both the infinite divisibility of space and the contrary.

The third shows the necessity, but also the impossibility of human freedom.

The fourth proves the existence of a necessary being and the lack of existence of such a being.

It can seem that the world must have a beginning or limit in space and time. After all, doesn't everything have a beginning and/or end? But it can also seem that the world must be infinite in space and time. After all, what would be beyond the limit of space, or before time began?

Kant argued that his distinction between noumena and phenomena helps to avoid such paradoxes. The Noumenal reality is purely intelligible and nonsensual, whereas the Phenomenal reality is the world as we experience it.

Space and time are not things in themselves (noumena) nor are they objects we experience (phenomena). They are forms that our experience takes or features of the way in which our minds present the world to us. To talk about the beginning or limit of space or of time is to mistakenly treat space and time as if they were objects of some kind.

Similarly, we can be led to think that there can be free will in the world, but also that there can be no such thing, since all events in the natural world are determined, i.e. caused by prior events. It is indeed a law of nature that all events are determined, Kant said, but this applies only to phenomena. In the realm of noumena it is entirely possible that there is real freedom, and indeed ethics require that we believe in such freedom of the will.