This is a continuation of this question c++ function ptr in unorderer_map, compile time error

I was trying to use std::function instead function pointer, and I can insert function only if the functions are static. otherwise I will get following error

main.cpp:15:11: error: no matching member function for call to 'insert'

  map.insert(std::make_pair("one",&Example::procesString));

#include<string>
#include <unordered_map>
#include<functional>

namespace Test
{
 namespace Test
{
  class Example
  {
  public:
    Example()
    {

      map.insert(std::make_pair("one",&Example::procesString));
    }
    static void procesString(std::string & aString)
    //void procesString(std::string & aString) -> compiler error 
    {

    }
    static  void processStringTwo(std::string & aString)
    {

    }

    std::unordered_map<std::string,std::function<void(std::string&)>> map;
  };
}
}

int main()
{
  return 0;
}