Почему операторы описываются без static?

#include <iostream>
*
class Counter
{
public:
****Counter(int sec)
****{
********seconds = sec;
****}
****void display() 
****{
********std::cout << seconds << " seconds" << std::endl;
****}
****Counter operator + (Counter c2)
****{
********return Counter(this->seconds + c2.seconds);
****}
****int operator + (int s)
****{
********return this->seconds + s;
****}
int seconds;
};
*
int main()
{
****Counter c1(20);
****Counter c2(10);
****Counter c3 = c1 + c2;
****c3.display();********** // 30 seconds
****int seconds = c1 + 25;* // 45
*****
****return 0;
}

Counter operator + (const Counter &c1, const Counter &c2)
{
    return Counter(c1.seconds + c2.seconds);
}
int operator + (const Counter &c1, const int s)
{
    return c1.seconds + s;
}

Counter operator + (Counter c1, const Counter &c2)
{
    c1.seconds += c2.seconds;
    return с1;
}

auto operator+(const Counter &c1, const Counter &c2) -> Counter
{
    return Counter(c1.seconds + c2.seconds);
}