which form the material of a mental sentence (or 'thought') in the same sense in which articulated sounds form the material of a spoken sentence, or black marks on paper the material of a written sentence. When you are doing a calculation in arithmetic it is quite irrelevant whether you have before your mind the images of black numbers or of red numbers, or no visual picture at all. And even if it were empirically impossible for you to do any calculation without imagining black numbers at the same time, the mental pictures of those black marks could, of course, in no way be considered as constituting the meaning, or part of the meaning, of the calculation.
Carnap is right in putting great stress upon the fact (always emphasized by the critics of 'psychologism') that the question of meaning has nothing to do with the psychological question as to the mental processes of which an act of thought may consist. But I am not sure that he has seen with equal clarity that reference to ostensive definitions (which we postulate for meaning) does not involve the error of a confusion of the two questions. In order to understand a sentence containing, e.g., the words 'red flag', it is indispensable that I should be able to indicate a situation where I could point to an object which I should call a 'flag', and whose color I could recognize as 'red' as distinguished from other colors. But in order to do this it is not necessary that I should actually call up the image of a red flag. It is of the utmost importance to see that these two things have nothing in common. At this moment I am trying in vain to imagine the shape of a capital G in German print; nevertheless I can speak about it without talking nonsense, and I know I should recognize it if I saw the letter. Imagining a red patch is utterly different from referring to an ostensive definition of 'red'. Verifiability has nothing to do with any images that may be associated with the words of the sentence in question.
No more difficulty than in the case of the other side of the moon will be found in discussing, as another significant example, the question of 'immortality', which Professor Lewis calls, and which is usually called, a metaphysical problem. I take it for granted that 'immortality' is not supposed to signify never-ending life (for that might possibly be meaningless
