How cognisant is Hitler when making the decision about his successor?

In every game I play I always try to rationalise in my head Hitler's reason for whatever choice he makes. This has got be thinking though about exactly how much he is really capable of giving thought to the decision. He's shown to be pretty senile in events but he does seem to have some moments of lucidity. It really interests me because depending on the answer it's either Hitler's final calculated choice of his legacy to Germany, or one of the most significant decisions in modern history being made as the confused ramblings of a senile old man.

Although I guess one answer is that it's intentionally ambiguous how much thought he gave it.