Should a helper function go in the header or in the implementation file?

When writing a helper method for a class in C++, should it be declared as a private method in the class's definition in the header (.h) file? For example:

/*Foo.h*/
class Foo {

     public:
         int bar();

     private:
         int helper();
};
...
/*Foo.cpp*/
...
int foo::bar() {
    int something = this->helper();
}

int foo::helper() {
    ...
}

Or alternatively, is it better not to declare it as a private member of the class, and instead just make it a free-standing function in the implementation?

/*Foo.h*/
class Foo {
    public:
        int bar();
};
...
/*Foo.cpp*/
...
int Foo::bar() {
    int something = helper();
    ...
}

int helper() {
    ...   
}