The Exclusion Principle
THE EXCLUSION PRINCIPLE plays an important role in quantum mechanics and has effects that are almost as profound and as far- reaching as those of the principle of relativity in classical physics. There are, in fact, striking similarities between the two, similarities with respect to the methodological plane on which they operate and with respect to the restrictive quality they exhibit. Both principles enact vetoes on a very basic level of physical description.1
Relativity demands that laws of nature which do not conform to its requirement of invariance shall be dismissed from consideration; the exclusion principle says that states (in the quantum sense) which fail to have certain mathematical properties are not realized in nature. Needless to emphasize, both principles are verified successfully in elaborate and painstaking ways, and the legal flavor of the preceding statement is unfortunate; yet it is a fact that relativity and exclusion guide research as procedural vetoes and not as descriptive maxims derived from experience.
Because they play this role, one is almost tempted to list these principles among the metaphysical requirements of Chap. 5. Certainly, they are more basic and more general in their application than the special laws of nature which they force into conformity with their demands. On the other hand, such demands are somewhat more specific than those of logical fertility, extensibility, causality, and so forth, and the discovery of both principles is so recent that their complete amalgamation with the more traditional methods of science has perhaps not taken place. And it should also be said that exclusion at present shows marks of be-____________________