The theory of relations presented in this article consists of the general and the special theory of relations. The formulae of the former are meaningful and its laws valid for relations of any type. The meaningfulness of expressions and the validity of laws of the latter are restricted to relations of a certain special type. The special theory of relations is distinguished from the applied theory of relations. The applied theory of relations is the general theory applied to relations of (...) a special type, the difference between the general and the applied being in the form of the variables. The special theory of relations and the applied theory, when concerned with relations of the same special type, may differ with respect to the form of constants without differing with respect to variables. (shrink)