at least unconsciously, and in the same way it has always been acknowledged by common sense in everyday life; the only place where it has been neglected is in philosophical discussions. Science could not possibly act otherwise, because its whole business consists in testing the truth of propositions, and they cannot be tested except on the strength of our principle.
Now and then it happens in the development of science that a concept is used in a vague manner so that there is no absolute clarity about the verification of the propositions in which the concept occurs. Within certain limits of accuracy the ordinary tests of their truth may suffice for years or centuries and then suddenly some contradiction will show itself and force the scientist to inquire carefully into the signification of his symbols. He will have to stop and think. He will pause in his scientific investigation and turn to philosophic meditations until the meaning of his propositions has become perfectly clear to him.
The most famous instance of this kind, and one which will forever be memorable, is Einstein's analysis of the concept of time. His great achievement, which is the basis of the Restricted Theory of Relativity, consisted simply in stating the meaning of assertions that physicists used to make about the simultaneity of events in different places. He showed that physics had never been quite clear about the signification of the term "simultaneity", and that the only way of becoming clear was to answer the question, "how is the proposition (two distant events happen at the same time) actually verified?" If we show how this verification is done we have shown the complete sense of the proposition and of the term, and it has no meaning besides. All those philosophers who have condemned Einstein's ideas and theory (and some are condemning it even to this day) do it on the ground that there is a simultaneity the signification of which is understood without verification. They call it "absolute simultaneity". This sounds very well, but unfortunately those philosophers have failed to tell us how their simultaneity can actually be distinguished from that of Einstein; they have not been able to give us the slightest hint how anyone can ever find out whether two distant events occur "absolutely simultaneously" or not. Considering this I think we must take the liberty of regarding their assertion as meaningless.
I have just alluded to the difference between the scientific attitude
