The phrase 'A Law of Nature' is probably rarer in modern scientific writing than was the case some generations ago. This is partly due to very natural objection to the use of the word 'law' in two different senses. Human societies have laws. In primitive societies there is no distinction between law and custom. Some things are done, others are not. This is regarded as part of the nature of things, and generally as an unalterable fact. If customs change, the change is too slow to be observed, later on kings could make new laws, but there was no way of getting rid of old ones. The Greek democracies made the great and revolutionary discovery that a community could consciously make new laws and repeal old ones. So for us a human law is something which is valid only over a certain number of people for a certain period of time. Laws of Nature, however, are not commands but statements of facts. The use of the same word is unfortunate. This would do away with the idea that a law implies' a law-maker. But the difference between the two uses of the word is fundamental. If a piece of matter does not obey a Law of Nature it is not punished. On the contrary, we say that the law has been stated incorrectly. Certainly many of them have. Nevertheless, these inaccurately stated laws are of immense practical and theoretical value. The main topic of this passage is that ______.