its sense? The answer is this: We know the meaning of a proposition when we are able to indicate exactly the circumstances under which it would be true (or, what amounts to the same, the circumstances which would make it false). The description of these circumstances is absolutely the only way in which the meaning of a sentence can be made clear. After it has been made clear we can proceed to look for the actual circumstances in the world and decide whether they make our proposition true or false. There is no vital difference between the ways we decide about truth and falsity in science and in every-day life. Science develops in the same ways in which does knowledge in daily life. The method of verification is essentially the same; only the facts by which scientific statements are verified are usually more difficult to observe.
It seems evident that a scientist or a philosopher when he propounds a proposition must of necessity know what he is talking about before he proceeds to find out its truth. But it is very remarkable that oftentimes it has happened in the history of human thought that thinkers have tried to find out whether a certain proposition was true or false before being clear about the meaning of it, before really knowing what it was they were desirous of finding out. This has been the case sometimes even in scientific investigations, instances of which I will quote shortly. And it has, I am almost tempted to say, nearly always been the case in traditional philosophy. As I have stated, the scientist has two tasks. He must find out the truth of a proposition and he must also find out the meaning of it, or it must be found out for him, but usually he is able to find it for himself. In so far as the scientist does find out the hidden meaning of the propositions which he uses in his science he is a philosopher. All of the great scientists have given wonderful examples of this philosophical method. They have discovered the real significance of words which were used quite commonly in the beginning of science but of which nobody had ever given a perfectly clear and definite account. When Newton discovered the concept of “mass” he was at that time really a philosopher. The greatest example of this type of discovery in modern times is Einstein's analysis of the meaning of the word “simultaneity” as it is used in physics. Continually, something is happening “at the same time” in New York and San Francisco, and although people always thought they knew perfectly well what was meant by such a statement
