Why does it check in base class, while it is already pointing to the derived class?

#include<iostream> 
using namespace std; 

class Base { 
private: 
    virtual void fun() { cout << "Base Fun"; }
}; 

class Derived: public Base { 
public: 
    void fun() { cout << "Derived Fun"; } 
}; 

int main() 
{ 
Base *ptr = new Derived; 
ptr->fun(); 
return 0; 
} 

The above function gives the error: ‘virtual void Base::fun()’ is private within this context

But why does it need to check in the base class, while in the declaration we are making it clear that it's pointing to derived class?