If all things are impermanent, then how can Buddhism make absolute assertions?

I had someone ask me this afternoon: > If Buddhism teaches that all things are impermanent, then how can it > make absolute assertions such as **there are 5 aggregates** or **there > are 4 noble truths**? Since I am very new to all of this I had to honestly answer, "I don't know". I would appreciate any insights on this question.