In all this I have been considering the question of the relation between the structure of language and the structure of the world. It is clear that anything that can be said in an inflected language can be said in an uninflected language; therefore, everything that can be said in language can be said by means of a temporal series of uninflected words. This places a limitation upon what can be expressed in words. It may well be that there are facts which do not lend themselves to this very simple schema; if so, they cannot be expressed in language. Our confidence in language is due to the fact that it consists of events in the physical world, and, therefore, shares the structure of the physical world, and therefore can express that structure. But if there be a world which is not physical, or not in space-time, it may have a structure which we can never hope to express or to know. These considerations might lead us to something like the Kantian _a priori_, not as derived from the structure of the mind, but as derived from the structure of language, which is the structure of the physical world. Perhaps that is why we know so much physics and so little of anything else. However, I have lapsed into mystical speculation, and will leave these possibilities, since, by the nature of the case, I cannot _say_ anything true about them. CHAPTER XXV THE VALIDITY OF INFERENCE It is customary in science to regard certain facts as “data”, from which laws and also other facts are “inferred”. We saw in Chapter VII that the _practice_ of inference is much wider than the theories of any logician would justify, and that it is nothing other than the law of association or of “learned reactions”. In the present chapter, I wish to consider what the logicians have evolved from this primitive form of inference, and what grounds we have, as rational beings, for continuing to infer. But let us first get as clear a notion as we can of what should be meant by a “datum”.