x) : aRx . xRb,
(x, y) : aRx . xRy . yRb, . . .
The general term of a formal series can only be expressed by a
variable, for the concept symbolized by "term of
this formal series" is a formal concept. (This
Frege and Russell overlooked; the way in which they express
general propositions like the above is, therefore, false; it
contains a vicious circle.)
We can determine the general term of the formal series by giving its first term and the general form of the operation, which generates the following term out of the preceding proposition.