parent class member in subclass using templates, without `this`

Question

Suppose I have the following hierarchy:

// the template of the class isn't important for
// the question, I'm using an enum in the real code
enum class State : int { S0, S1 /*... etc */ };

template<State S>
class Base {
public:
    Base(const Vector3f &position) : mPosition(position) {}
protected:
    Vector3f mPosition;
};

template<State S>
class Derived : public Base<S> {
public:
    Derived(const Vector3f &position) : Base<S>(position) {
        // vvvvvvvvv ERROR
        if(mPosition.norm() > 2.0f) { /* do stuff */ }
};

I am aware that I can use this->mPosition as well as Base<S>::mPosition, but I do not understand why. If templates were not involved, it would compile.

I believe this is because at compile time there is no proof that all possible Base<State S> specializations (if they exist) will have an mPosition. Is this correct?

In my particular scenario, I can guarantee that every version of Base and Derived will be compiled and that they will all also have mPosition. Is there anything I can do to use just mPosition in the Derived class?