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Laws of Nature, however, are not commands but statements of facts. The use of the same word is unfortunate. It would be better to speak of uniformities of Nature. This would do away with the elementary fallacy that a law implies a law-giver. Incidentally, it might just as well imply a parliament or soviet of atoms. 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 incorrectly stated, It is quite probable that every law of Nature so far stated has been stated incorrectly. Certainly many of them have. Nevertheless, these inaccurately stated laws are of immense practical and theoretical value. They fall into two classes-qualitative laws such as All animals with feathers have beaks, and quantitative laws such as Mercury has 13,596 times the density of water. The first of these is a very good guide. But it was probably not true in the past. For many birds which were certainly feathered had teeth and may not have had beaks. And it is quite possibly not today. There are about a hundred thousand million birds on our planet, and it may well be that two or three of them are freaks which have not developed a beak. But have lived long enough to grow feathers. It was thought to be a law of Nature that female mammals had mammary glands, until Prof. Crew of Edinburgh found that many congenitally hairless female mice lacked these organs, though they could bear young which other females could then foster. | 
