And thus we come to numbers: I define

x = ⁰'x  Def.   e
'^v'x = ^v+1'x  Def.

According, then, to these symbolic rules we write the series 
x, 'x, ''x, '''x, . . . . . 
as: ⁰'x, ⁰⁺¹'x, ⁰⁺¹⁺¹'x, ^0+1+1+1'x, . . . . .

Therefore I write in place of "[x, , ' ]" - scrivo: 

"[⁰', ^v'x, ^v+1'x]",

And I define:
0 + 1 = 1  Def.
0 + 1 + 1 = 2  Def.
0 + 1 + 1 + 1 = 3  Def.
and so on.

6.021    A number is the exponent of an operation.

6.022    The concept of number is nothing else than that which is common to all numbers, the general form of a number.

The concept of number is the variable number.