The question has been answered but I thought a self-evident example would still help:

1. Whatever is alive is not dead.
2. Whatever is dead is not alive.
3. I am alive.
4. You are not me.

Conclusion: you must be dead.

The question has been answered but I thought a self-evident example would still help:

1. Whatever is alive is not dead.
2. Whatever is dead is not alive.
3. I am alive.
4. You are not me.

Conclusion: you must be dead.

This obviously isn't the case. Why not? Well, simply put: just because I am alive (3), does not mean that I am the only one who is alive.

This defeats the conclusion that anyone who is not me must invariably be dead.

I am one of the creatures who are alive. I am not (conclusively proven to be) the only one.

Similarly, the phenomenon of cats being born into this world is one of the things that are natural. It is not (conclusively proven to be) the only natural thing in this world.

This defeats the conclusion that anything other than the phenomenon of cats being born must invariably be unnatural.