that a journey to another celestial body were absolutely incompatible with the known laws of nature, a proposition about the other side of the moon would still be meaningful. Since our sentence speaks of certain places in space as being filled with matter (for that is what the words ‘side of the moon’ stand for), it will have meaning if we indicate under what circumstances a proposition of the form, ‘this place is filled with matter’, shall be called true or false. The concept ‘physical substance at a certain place’ is defined by our language in physics and geometry. Geometry itself is the grammar of our propositions about ‘spatial’ relations, and it is not very difficult to see how assertions about physical properties and spatial relations are connected with ‘sense-data’ by ostensive definitions. This connection, by the way, is not such as to entitle us to say that physical substance is ‘a mere construction put upon sense-data’, or that a physical body is ‘a complex of sense-data’ — unless we interpret these phrases as rather inadequate abbreviations of the assertion that all propositions containing the term ‘physical body’ require for their verification the presence of sense-data. And this is certainly an exceedingly trivial statement.
In the case of the moon we might perhaps say that the meaning-requirement is fulfilled if we are able to ‘imagine’ (picture mentally) situations which would verify our proposition. But if we should say in general that verifiability of an assertion implies possibility of ‘imagining’ the asserted fact, this would be true only in a restricted sense. It would not be true in so far as the possibility is of the empirical kind, i.e., implying specific human capacities. I do not think, for instance, that we can be accused of talking nonsense if we speak of a universe of ten dimensions, or of beings possessing sense-organs and having perceptions entirely different from ours; and yet it does not seem right to say that we are able to ‘imagine’ such beings and such perceptions, or a ten-dimensional world. But we must be able to say under what observable circumstances we should assert the existence of the beings or sense-organs just referred to. It is clear that I can speak meaningfully of the sound of a friend's voice without being able actually to recall it in my imagination. — This is not the place to discuss the logical grammar of the word ‘to imagine’; these few remarks may caution us against accepting too readily a psychological explanation of verifiability. We must not identify meaning with any of the psychological data
