Impermanence is Buddha nature / Buddha nature is impermanence Can anyone please explain these aphorism / these aphorisms?

Thank you in advance.
I don't know, so the following is a complete guess, and I hope someone will give a better answer.

Apparently the aphorism come from Dōgen.

Dōgen has a perhaps-non-standard view of Buddha-nature, which he calls Busshō.

My guesses are:


If according to Dōgen everything (real) is Buddha-nature then (because everything real is impermanent), therefore everything (i.e. Buddha-nature) is impermanent.
So for example any description of Buddha-nature and anyone describing Buddha-nature and anything to which the term Buddha-nature could be applied is impermanent.
As for the converse (i.e. impermanence is Buddha-nature), is that related to the third noble truth, or to rise and fall?