Different logical types can have nothing whatever in common. But the same fact that we can talk of the possibility of a relation of n places, or of an analogy between one with 2-places & one with 4, shews that relations with different numbers of places have something in common, that therefore the difference is not one of type but like the difference between different names – something which depends on experience. This answers the question how we can know that we have really got the most general form of a proposition. We have only to introduce what is common to all relations of whatever number of places.