The phrase "A law of Nature" is probably rarer inmodern scientific writing than was the case somegenerations ago. This is partly due to a very naturalobjection to the use of the word "law" in twodifferent senses. Human societies have laws. Inprimitive societies there is no distinction betweenlaw and custom. Some things are done, others arenot. 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 beobserved. Later on kings and prophets could promulgate new laws, but there was no way ofrevoking old ones. The Greek democracies made the great and revolutionary discovery that acommunity could consciously make new laws and repeal old ones. So for us a human law issomething 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 sameword is unfortunate. It would be better to speak of uniformities of Nature. This would do awaywith the elementary fallacy that a law implies a law-giver. Incidentally, it might just as wellimply a parliament or soviet of atoms. But the difference between the two uses of the word isfundamental. If a piece of matter does not obey a law of Nature it is not punished. On thecontrary, we say that the law has been incorrectly stated, It is quite probable that every law ofNature 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"(at 0°C and 1atmosphere's pressure). The first of these is a very good guide. But it was probably not truein the past. For many birds which were certainly feathered had teeth and may not have hadbeaks. And it is quite possibly not today. There are about a hundred thousand million birds onour planet, and it may well be that two or three of them are freaks which have not developed abeak. But have lived long enough to grow feathers. It was thought to be a law of Nature thatfemale mammals (defined as warm-blooded vertebrates with hair) had mammary glands, untilProf. Crew of Edinburgh found that many congenitally hairless female mice lacked these organs,though they could bear young which other females could then foster.

　　And quantitative laws generally turn out to be inexact. Thus water is nothing definite. Itis a mixture of at least six different substances. For in the molecule H20, one or both of thehydrogen atoms may be either light or heavy, and so may the oxygen atom. Similarly, mercuryconsists of several different types of atom. Thus the ratio of the densities of mercury and wateris not fixed, though in the case of ordinary samples the variation is too small to be detected.But it can be detected if the water happens to have been taken from an accumulator which hasbeen used for some time.

　　In his theory of Probability (Oxford, 1939) Jefferys has something new to say aboutinduction. Two contradictory theories are in vogue as to the laws of Nature. The older view isthat they are absolute, though of course they may have been inaccurately formulated. Theextreme positivistic view, enunciated by Vaihinger, is that we can only say that phenomenaoccur as if certain laws held. There is no sense in making any definite statements, though it isconvenient to do so.

　　Now Jeffreys points out that, if a number of observations have been found to conform to alaw, it is highly probable that the next one will do so whether the law is true or not. In Jeffrey'swords? "A well-verified hypothesis will probably continue to lead to correct inferences even if itis wrong. "

　　Positivists and idealists have made great play with the fact that many laws of Nature, asformulated by scientists, have turned out to be inexact, and all may do so. But that isabsolutely no reason for saying that there are no regularities in Nature to which ourstatements of natural law correspond. One might as well say that because no maps of Englandgive its shape exactly it has no shape.

　　What is remarkable about the laws of Nature is the accuracy of simple approximations.One might see a hundred thousand men before finding an exception to the rule that all menhave two ears, and the same is true for many of the laws of physics. In some cases we can seewhy. The universes is organized in aggregates, with, in many cases, pretty wide gaps betweenthem. Boyle's law that the density of a gas is proportional to its pressure, and Charles' lawthat the volume is proportional to the temperature, would be exact if gas molecules werepoints which had no volume and did not attract one another. These laws are very nearly true forgases at ordinary temperatures and pressures, because

　　the molecules occupy only a small part of the space containing the gas, and are closeenough to attract one another only during a very small part of any interval of time. Similarly,most of the stars are far enough apart to be treated as points without much error when we areconsidering their movements.

　　And most men manage to protect themselves from injury so far as is needed to keep bothears. Whereas trees cannot protect themselves form the loss of branches. It is very rare tosee a completely unmutilated, and therefore completely regular tree. Mendel's laws, according towhich two types occur in a ratio of 1： 1 in some cases and 3 ： 1 in others, are theoreticallytrue if the processes of division of cell nuclei are quite regular, and if neither type is unfit so asto die off before counts are made. The first condition never holds, and the second probablynever does. But the exceptions to the first condition are very rare. In one particular case acritical division goes wrong about one in ten thousand times. The effect of this on a 1 : 1 ratioor 3 ! 1 ratio could be detected only by counting several hundred million plants or animals.Differences in relative fitness are more important. But even so the Mendelian ratios aresometimes fulfilled with extreme accuracy, and are generally a good rough guide.

　　Jeffreys points out that in such cases it is often much better to stick to the theoretical lawrather than the observed data. For example, if you are breeding silver foxes and a new colourvariety occurs which, if crossed to the normal, gives 13 normal and 10 of the new colour, youare much more likely to get a ratio of about 1:1 than 13:10 if you go on with such matting,even though if you breed many thousands the 1: 1 ratio will not hold exactly. The mathematicaltheory which Jeffreys has developed concerning such cases is particularly beautiful, but canhardly be summarized here.

 

　　1. Ordinarily, gas molecules are so close that they attract one another for only a very shorttime.

　　2. The statement that atoms in the molecule H2O may be light or heavy is a sample ofquantitative laws.

　　3. Human law is similar to natural law in essence.

　　4. Charles' law and Boyle's law are based on observations made at ordinary temperaturesand pressures.

　　5. Since cell-division is sometimes irregular and certain types die off early, we sometimesget neither 3 ： 1 ratio nor 1 : 1 ratio.

　　6. Differences in relative fitness are more frequent than irregular cell-division.

　　7. We must see many human beings before stating the rule that all men have 2 ears.

　　8. Compared with human laws, laws of nature are accurate because they are expressed inthe form of_______.

　　9. In considering the movements of stars, scientists need not consider their_______.

　　10. _______laws can serve as good guides.

 

参考答案：

　　I. Y 2. Y 3. N 4. N 5. Y 6. NG 7. N 8. approximation 9. distance 10. Qualitative