And the Buddha also offers 4 kinds of exercises in order to help you look into the object of mind. The 13th exercise is the contemplation of impermanence. That is one of the practices of contemplation. But many of us only have an idea of impermanence, a notion of impermanence, an intellectual understanding of impermanence. That's not impermanence. Because even if you agree that everything is impermanent, you still behave as if they are permanent. So, impermanence here is an insight and not a notion. A notion does not have power to liberate you. Only an insight (have power to liberate you). The notion can be helpful but you have to be careful not to be caught in notions, even the notion of impermanence, notion of no-self, notion of interbeing.
These notions (impermanence, no-self, interbeing) are the skillful means in order to help you. You are given some instruments to work the land in order to grow vegetable. And that instrument you should not worship it, you should not put into an alter and worship it. You should make good use of that instrument in order to work the land, in order to produce vegetables. So, the teaching is given not for you to worship, not for you to say, "I have the truth. I monopolize the truth". That truth is called "interbeing", "impermanence", "non-self". It does not help. So, the practice of "Samadhi", concentration is to produce not a thought, not a notion, but an insight. It is the insight of impermanence that helps liberate you, not a notion of impermanence.
You know that your beloved one is impermanent. But you still have the feeling that she will be there for one thousand years. And if you know that she is impermanent, you do anything that can make her happy right here and right now. Maybe tomorrow it's too late. So, the idea of impermanence does not help. Only the insight of impermanence can help liberate us. Suppose you are angry at your beloved one. She just said something that makes you unhappy. She just did something that makes you unhappy. And with the anger in you, you want to suffer less. You want to say something in order to make her suffer. And you believe that (by) doing that, you will suffer less. You are not the practitioner. That's why when you suffer, you want to punish to make another person suffer so that you get a relief. But that is an escalation of your suffering. If you say something strong, mean to make him or her unhappy, she will suffer and she will try to suffer less by saying back something that makes you suffer. Punishment. And we know that it is an escalation of anger.
But according to this (exercises on mindful breathing), you can do otherwise. When you get angry with your beloved one, and when you are about to say something to make her suffer, you remember that you are practitioner. You close your eyes, you breathe in, and you visualize your beloved one 300 years from now. Watch my beloved one become in 300 years. And you don't need a lot of time. A few seconds can bring you an insight of impermanence. And you can realize right away that to get angry with each other like this, is stupid. What I will become in 300 years? What she will become in 300 years? That's not wise to make each other suffer like that. Maybe you need only one in-breath in order to touch the nature of impermanence in you and in her. And that is insight, and not what you talk about.
And when you breath out, you open your eyes. And only thing you want to do now is to take her into your arms and practice mindful breathing. Breathing in, I know she is still alive. And breathing out, I am so happy. The transformation can be very quick with concentration. Concentration is Samadhi. So, impermanence should not be a notion or idea. It should be a practice, be a concentration. And the first concentration that the Buddha recommends in the Sutra on Mindful Breathing is the concentration on impermanence.
There is a big difference between notion or idea and insight. Ego (separate self) thinks by using notions or ideas, without a practice. Meanwhile, awareness (non-separate self) attains insight without thinking, and will practice according to the insight. So, the key is who we are. In order to transform oneself from ego to awareness, we need to stop thinking through mindfulness. Mindful breathing will help us stop thinking. If we are good practitioners, one in-breath is enough to revive awareness (non-separate self). But this is still the stage of the part-time Buddha. In order to become the full-time Buddha, we need to throw away all notions by understanding the ultimate truth. Nirvana is the extinction of all notions.