him that, if this is the case there is no meaning in his propositions about a metaphysical external world and that we must continue to use this phrase in the good old innocent sense in which it stands for stars, mountains, and trees, as contrasted with dreams, feelings and wishes which form the "internal" world. We must inform the philosopher that it is not his business to tell us what is real and what is unreal — this must be left to experience and science — but it is his business to tell us what we mean when we judge of a certain thing or event that it is "real". And in every case he can answer the question concerning the sense of such a judgment only by pointing to the operations by which we should actually verify its truth. If I know exactly what I have to do in order to find out whether the shilling in my pocket is real or imagined, then I know also what I mean by declaring that the shilling is a real part of the external world and there is no other meaning of the words "real" or "external world".
For, let us repeat it once more: the complete and only way of giving the meaning of a proposition consists in indicating what would have to be done in order to find out if the proposition is true or false (no matter whether we are actually able to do it). This insight is often called "the experimental (or operational) theory of meaning" but I should like to point out that it would be unjust to call it by such an imposing name. A "theory" consists of a set of propositions which you may believe or deny, but our principle is a simple triviality about which there can be no dispute. It is not even an "opinion", since it indicates a condition without which no opinion can be formulated. It is not a theory, for its acknowledgement must precede the building of any theory. A proposition has no meaning unless it makes a discoverable difference whether it is true or false; a proposition whose truth or falsity would leave the world unchanged, does not say anything about the world, it is an empty sentence without meaning. "Understanding" a proposition means: being able to indicate the circumstances which would make it true. But we could not describe these circumstances if we were not able to recognise them, and if they are recognisable it means that the proposition is in "principle" verifiable. Thus understanding a statement and knowing the way of its verification is one and the same thing.
This principle is nothing surprising or new or wonderful: on the contrary, it has always been followed and used by science as a matter of course,
