An odd C++ error: test.cpp:15: error: passing ‘const *’ as ‘this’ argument of ‘*’ discards qualifiers

I'm having some trouble with a particular piece of code, if anyone can enlighten me on this matter it would be greatly appreciated, I've isolated the problem down in the following sample:

#include <iostream>

using namespace std;

class testing{
   int test();
   int test1(const testing& test2);
};

int testing::test(){
   return 1;
}

int testing::test1(const testing& test2){
   test2.test();
   return 1;
}

So what could possibly have cause the following error:

test.cpp:15: error: passing ‘const testing’ as ‘this’ argument of ‘int testing::test()’ discards qualifiers

Thanks a lot!