When we say of a proposition of form aRb that what symbolises is that ‘a’ “R” is between “a” & “b”, it must be remembered that in fact the proposition is capable of further analysis because a, R, & b are not simples. But what seems certain is that when we have analysed it, we shall in the end come to propositions of the same form in respect of the fact that they do consist in one thing being between 2 others.
     How can we talk of the general form of a proposition, without knowing any unanalysable propositions in which particular names & relations occur? What justifies us in doing this is that though we don't know any unanalysable propositions of this kind yet we can understand what is meant by a proposition of the form (∃x,y,R). xRy (which is unanalysable), even though we know no proposition of the form xRy.
     If you had any unanalysable proposition in which particular names & relations occurred (and an unanalysable proposition = one in which only fundamental symbols = ones not capable of definition, occur) then you always can form from it a proposition of the form (∃x,y,R). xRy, which though it contains no particular names & relations, is unanalysable.