ABSTRACT
The advance of any science is punctuated as much by the disappearance of old problems as by the emergence of new ones. This is little better than a truism if we have in mind problems disappearing and discussions or controversies ceasing because the issues in question have been resolved. But often it is not a question of solution; rather it is a question of changes of viewpoint and interest. The old problems are abandoned because they no longer seem important; the controversies cease because all that can be said has been said; and if certain questions still remain unanswered they are yet shelved in spite of it, or perhaps because of it—because one realizes that they are unanswerable and should be replaced by other, more profitable, ones.
