When one uses nickels to calibrate a digital scale, he assumes five nickels together weigh 25 grams. The question that follows is this: what kind of instrument can guarantee the precision and accuracy of a nickel's weight? Eventually, he will trace the standard of a gram to the IPK, then he discovers that the definition of a gram depends on temperature, length, atmospheric pressure and the purity of water. Next he hopes that those instruments that guarantee the precision and accuracy of T, L, P do not use components that are weight sensitive, and the definitions of T, L, P standard units do not depend on weight.
If a theory cannot stand on its own feet, it has no right to talk about other theories. Gödel used numbering to gauge PM without first explaining what numbers are. Gödel was a platonist, W&R were not; that was why Russell said that he and Gödel "never arrived at common premises from which to argue."(See Autobiography)
PM stands on its own feet. Although it was not intended to be a theory about other theories, you can use it to gauge other theories whenever it is applicable. In 2016, in this world of formalists, PM is very applicable.
In the department of philosophy and mathematics new branches sprout as prolifically as lianas in the rain forest and there is a pullulation of techniques, terms and symbols producing a heterogeneous crop full of chaff that conceals a few kernels of wisdom.
--Hilton, Alice Mary. Logic, Computing Machines, and Automation. Cleveland and New York: Meridian Books, 1964