A thing to be understood

It is a deep realization; that of passing away.

From the movie Troy:

Achilles [to Briseis]: I'll tell you a secret, something they don't teach you in your temple. The gods envy us. They envy us because we're mortal, because any moment might be our last. Everything is more beautiful because we're doomed. You will never be lovelier than you are now, and we will never be here again.

It seems to blend in with the Four Noble Truths.

1. Life means suffering.

2. The origin of suffering is attachment.

3. The cessation of suffering is attainable.

4. There is a path to the cessation of suffering.

Explained in more detail here: The Four Noble Truths