Can I implement a class from a separate namespace?

Question

For example, if I have a namespace, A, with a class, X, can I create a class (in a different namespace, B) that implements X?

Here is the code version:

namespace A {
class X {
}
}

namespace B {
class Y : X { // can I do this?
}
}

If I can't do this, then could I please get an explanation as to why?