verifying before you know the meaning, i.e., before you have established the possibility of verification.
In other words, the possibility of verification which is relevant to meaning cannot be of the empirical sort; it cannot be established post festum. You have to be sure of it before you can consider the empirical circumstances and investigate whether or no or under what conditions they will permit of verification. The empirical circumstances are all-important when you want to know if a proposition is true (which is the concern of the scientist), but they can have no influence on the meaning of the proposition (which is the concern of the philosopher). Professor Lewis has seen and expressed this very clearly (loc. cit. 142, first six lines), and our Vienna positivism, as far as I can answer for it, is in complete agreement with him on this point. It must be emphasized that when we speak of verifiability we mean logical possibility of verification, and nothing but this.
I call a fact or a process ‘logically possible’ if it can be described, i.e., if the sentence which is supposed to describe it obeys the rules of grammar we have stipulated for our language. (I am expressing myself rather incorrectly. A fact which could not be described would, of course, not be any fact at all; any fact is logically possible. But I think my meaning will be understood.) Take some examples. The sentences, ‘My friend died the day after tomorrow’; ‘The lady wore a dark red dress which was bright green’; ‘The campanile is 100 feet and 150 feet high’; ‘The child was naked, but wore a long white nightgown’, obviously violate the rules which, in ordinary English, govern the use of the words occurring in the sentences. They do not describe any facts at all; they are meaningless, because they represent logical impossibilities.
It is of the greatest importance (not only for our present issue but for philosophical problems in general) to see that whenever we speak of logical impossibility we are referring to a discrepancy between the definitions of our terms and the way in which we use them. We must avoid the severe mistake committed by some of the former Empiricists like Mill and Spencer, who regarded logical principles (e.g. the Law of Contradiction) as laws of nature governing the psychological process of thinking. The nonsensical
