Stuck at this c++11 Thread code for few days. Please suggest

I very recently started coding in C++11. Since I understood theoritical part, I thought of experimenting by writing code. This is my first program in C++, what it does is - I am creating a class which has a thread type member and 2 member functions, of which one is a thread function and the other is a function which starts this thread function. But I am facing the below error. I did a lot of Googling, but none of them helped. I want to spawn 2 threads of "runThread" and they will update the counter. I know the counter is not synchronised, but i will take care of that using std::atomic variable once this error gets resolved.Please advice.

#include <iostream>
#include <stdio.h>
#include <thread>
#include <vector>

class Application {
public:
    Application(int val): counter(val) { printf("value is %d\n", counter); }
    void start();
    void print();
private:
    void runThread();
    int counter;
    std::vector<std::thread> thr;
};

void Application::runThread() {
    for(int i=0;1<50;i++)
        counter++;
}

void Application::start() {
    //std::thread t1(runThread);
    //std::thread t2(runThread);
    //this->thr.emplace_back(std::thread(&Application::runThread, this)); Tried this, but dint work
    //this->thr.emplace_back(std::thread(&Application::runThread, this));Tried this, but dint work
    thr.emplace_back(std::thread(runThread));
    thr.emplace_back(std::thread(runThread));
}

void Application::print() {
    printf("Counter = %d\n", counter);
}

int main(void)
{
    int a;
    Application app(300);
    app.start();
    std::cin >>a;
}

This is the error

../main.cpp: In member function ‘void Application::start()’:
../main.cpp:27:40: error: invalid use of non-static member function
  thr.emplace_back(std::thread(runThread));
                                        ^
../main.cpp:28:40: error: invalid use of non-static member function
  thr.emplace_back(std::thread(runThread));
                                        ^
subdir.mk:18: recipe for target 'main.o' failed
make: *** [main.o] Error 1