Why does the mind always want to 'do something' and how to cultivate 'non-doing'?

Maybe the tragedy of human life is that we cannot stop thinking.

Meditation can be useful for getting some distance from our thinking.

Nagarjuna came up with this non-doing, if I remember right. I think this is profound, disturbing.

The mind is conditioned. It got this way over a long time. It keeps us alive, usually.

Non-doing does not mean simply sitting doing nothing. Non-doing means you put yourself and more with full involvement/intensity into that activity that you do, (even sitting), so much so that you forget you exist. Like Flow. Only that action and the activity is going on.

This "you" is an identity created for social/survival purposes according to karmic tendencies/compulsions. This body is also chosen for that presence. Until these tendencies/compulsions go away, you cannot simply sit.

Always wanting to do something is because somewhere inside, we are/feel "incomplete". Do you observe? When you buy something or finish doing something, you feel a little relaxed and then sometime later, the mind comes up again, the body comes up again, needing to do something. This is because of our karma/karmic tendencies. We don't feel complete. That's the only problem. Until we finish our karmic bag, we are going to be here trying to do something all the time.

This question is very significant and is really where all seekers are tripping .You said something very significant ,

How can I train my mind to just be?

.Its important to understand that identification is the sole reason for doing.Every part of your body is doing something ,your organs are functioning ,you don't have to make your heart beat or your stomach digest the food ,they work by themselves.But what we fail to notice is that the mind is also an organ .If I say Iam just my stomach then I will feel responsible for its success and failure .Iam identified with it ,same is our identification with the mind ,from childhood we are told to concentrate or to memorize something,when these are really just mind functions ,and we are not the mind.

So removing the identification with mind is necessary .its also important to understand that just like a computer game, the mind creates an object called truth or non-doing ,a distance or a path and a virtual character seeker who wants to get there .Recognizing this whole play or matrix is meditation.Vipassana is a methodological way of dissecting the game and watching it all with every seeing there is a dis-identification,The non-doing is not the mind's idea of non-doing ,its so simple there is no distance to it ,because distance is also mind creation,lastly identification with the seeker is gone and now the mind can do and play its game just like your other organs and there is no identification with a you that has to do anything because it was simply a virtual character created by mind.

You ask: How can I train my mind to just be?

Show me this 'I'. What is 'my mind'?

You cannot show me because there is no 'I'. If there was, no-one would have bad habits, because their 'I' would stop them.

Keep on with Zazen. If you were in a Rinzai monastery like I was, you would have to solve a 'Ko an', such as 'what was my original face before I was born? Rather than thinking about being try to solve this Ko an.

That twinge you feel to "do something"? That's your small mind making a desperate attempt to remind you that it's in charge. If you listen to it, it will continue to be your master. If you really want to wake up, you have to stop letting it rule you. This is hard. Make no mistake about it. Buddhism calls this process sila or moral discipline, but other traditions use words like "mortification" or "purification". If those words sound harsh, then they are hitting the mark. This is not a comfortable process and it can involve monumental, willful efforts to modify, soften, and expunge negative patterns of behavior in body, speech, and mind. In the modern world, this means getting off the internet, putting down your phone, and restricting your intake of negative influences (media, intoxicants, negative people, etc.). In Zen, we'd moreover advise that you sit down on the cushion and bask in that discomfort. Don't worry so much about watching the breath or entering samadhi just yet. Instead, when those urges to "do something" come, just stop what you are doing and let them wash over you. Let those urges kill you. Trust the process and you will eventually come out the other side better prepared to ignore the dictates of your small mind. In fact, those very dictates will begin to lessen.

Once the demands of your small mind begin to abate, I promise that you will begin to hear another another voice calling from a deeper place. At first it will be quiet, distant, and unintelligible. If you continue to apply yourself with moral discipline, it will become louder and clearer. When you've reached the place when your small mind in no longer pulling you this way and that, you are ready to add samadhi practice. The rest of the way up the mountain involves using a mind made clear and stable by zazen to focus in on that deeper voice and really learn what it has to teach.

Committing wholeheartedly to what we do each moment by moment, we experience increasing clarity and peace.

SN35.95:10.1: “In that case, when it comes to things that are to be seen, heard, thought, and known: in the seen will be merely the seen; in the heard will be merely the heard; in the thought will be merely the thought; in the known will be merely the known.

If necessary, observe your breaths (e.g., counting is a way of observing) as you engage with everyday tasks. Your breaths repeat and sustain no matter the distraction. When I started Zen, I was told to count my breaths. That was four decades ago. I still count my breaths. The distractions come and go. Greet each distraction with an acknowledgement and let each fade away without greed, aversion or delusion.

MN10:4.3: Just mindful, they breathe in. Mindful, they breathe out.

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To address the "cultivate non-doing" part specifically:

Wu-wei (not-doing) is not the practice of literally not doing absolutely anything (is that even possible?), it is the practice of un-doing the samsaric mind with its patterns and stereotypes.

For example, to stop the inner monologue is not-doing. To stop thinking in terms of, "I am the doer" is not-doing. To stop thinking in terms of "I am a human being" is not-doing. To stop thinking in terms of doing vs not doing is not-doing. To stop seeking sustenance in enjoyable activities is not-doing. To stop defending one's ego is not-doing. Etc.

Imagine for a moment that you picked up a hammer from your toolbox, and then forgot that you were holding it. You'd walk around all day with the hammer in your hand: You'd open doors with it, use it to shift gears in your car, poke people with it to get their attention, tap it on the table when you're bored, use it (somehow) to eat your food... If you have a forgotten hammer in our hands, you'll use it for all sorts of things whether it's useful or not, merely because you have it in your hand.

The thinking mind is a tool we use to solve problems. That's what it does. Unfortunately, most of us don't remember that we're holding it, most of the time, and we don't know how to put it down when we don't need it. And so we find ourselves using our mind for things it's not really useful for, or to entertain ourselves when we're bored, or whatever. It's always there, something we're clutching without realizing it, and using for no other reason than that we're clutching it.

That's the value of meditation. It teaches us to put the tool of the mind down, by creating a context where we just don't need to use the mind at all. But whether or not you're in meditation, if you feel an irrepressible urge to do something or think something, ask yourself whether you actually need to use that tool at just that moment. If you don't, then stop. You have to keep reminding yourself that the thinking mind is a tool you use when you want or need to, not a thing constantly held at the ready.