Pointer compatibility. Why it's forbidden to assign T** to const T**?

Question

Trying to get the pointer compatibility straight. Here's a code sample demonstrating the thing I'm finding hard to comprehend:

struct T
{
    int dummy;
};

int main(int argc, const char * argv[])
{
    T t;
    const T* cpt = &t; //Ok.

    const T** ppct = &cpt; //Ok.

    T* pt = &t;
    const T** ppct2 =&pt;
    //The line above gives me
    //Cannot initialize a variable of type 
    //'const T **' with an rvalue of type 'T **'

    return 0;
}

Before the return statement I'm trying to get the address of pointer to T, and assign it to the pointer to pointer to const T. What can go wrong, right? I won't be able to change the object I'm pointing to, but I'm not allowed to do this assignment.