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     The indefinables of logic must be independent of each other. If an indefinable is introduced, it must be introduced in all combinations in which it can occur. We cannot therefore introduce it first for one combination, then for another; e.g., if the form xRy has been introduced, it must henceforth be understood in propositions of the form aRb just in the same way as in propositions such as (∃x,y). xRy & others. We must not introduce it first for one class of cases, then for the other; for it would remain doubtful if its meaning was the same in both cases, & there would be no ground for using the same manner of combining symbols in both cases. In short, for the introduction of indefinable symbols & combinations of symbols the same holds, mutatis mutandis, that Frege has said for the introduction of symbols by definitions.