If, for example, we suppose that the function
F(fx) could be its own argument, then there would be a
proposition "F(F(fx))", and
in this the outer functions F and the inner function F
must have different meanings; for the inner has the form
(fx),
the outer the form
((fx)).
Common to both functions is
only the letter "F", which by itself
signifies nothing.
This is at once clear, if instead of
"F(F(u))" we write
"(
) : F(u) . u=Fu".
Herewith Russell's paradox vanishes.