A necessary aspect of having inner peace is fully and unconditionally accepting and allowing the outer world that currently exists. That present outer world is an unchangeable aspect of eternal reality in the same way the past is. Thus, in the same way one must fully and unconditionally accept and allow the past to be the way it is to have inner peace, one must also fully and unconditionally accept and allow the outer present to be the way it is.
What has already happened in what you call your present is as unchangeable as what you call the past. To realize this is to see and accept the eternal unchangeable is-ness of all of what is (and all of what was and all of what inevitably will be). In other words, it is to accept the inexorable fact that, whatever it is, it is what it is, and it already is. And, thus, it is an unchangeable aspect of eternal reality in the same way the past itself is.
In other words, as soon something appears to you in your present it is already part of unchangeable eternal reality in the same sense the past is.
To have inner peace, you must learn to unconditionally allow, accept, and love what has been revealed to you as an unchangeable aspect of eternal reality, which includes all of what you call the past and includes the entire outer present, which means essentially all of the present. From this perspective, you don't so much see things out your control as actually happening but rather as becoming revealed, like the second half of a movie you are in the middle of watching, or like looking at the cards you have already been dealt in a card game.
In a sense, you can choose (a.k.a. change) your present inner attitude about the past and present, namely in terms of lovingly accepting it versus hatefully resenting it, but you can't change the past or present itself (i.e. the outer world). In other words, the present outer world is essentially already the past, and thus it as utterly unchangeable as the past. Thus, in the same way inner peace necessarily requires fully and unconditionally accepting the past, it also requires fully and unconditionally accepting the outer present.
It's worth noting that this 'outer present' doesn't merely include everything that presently exists outside of the human skin of your human body. It does indeed include that, but it also includes much more. The reason is that the 'inner' in 'inner' peace doesn't refer to inside your human skin, but rather it means spiritually inside. It's not a spatial inside (i.e. an inside in space). Rather, it's a reference to the real you. In other words, it refers to that which is within your ability to consciously choose in your present, or in yet other words under the control of your present conscious will in your present. This may be much more limited than you think; It may be much more like watching a movie than playing a video game.
At least most of what goes on inside the boundary set by your human skin is entirely out of your control in the present and is not a matter of your conscious choice, the same as in the more literally outer world. In other words, most of what occurs inside the skin of what you call your human body is part of what this post refers to as the outer present. Most of it is not something you are presently choosing. Most of it is not something you are presently willing with your present conscious will.
For example, whether or not your human body has cancer right now is not a matter of your present choice. Whether or not your heart just beat just this second right now is not a matter of your choice in your present, hence why you probably don't say things like, "I beat my heart."
The bodily emotions you are experiencing are generally not a matter of your choice in your present, but rather are an unchangeable aspect of the current time and place in which you find yourself. They are generally a necessary result of the past, not your present choice in your present. The choices of your past self, and any necessary effects resulting from those choices (necessarily in the sense that they cannot be changed by your choices now in the present), are not in your control, are not something you are choosing, and are aspects of unchangeable eternal reality in the same sense the past is. For example, if your so-called past self recently chose to stick his or her hand in a fire and get severely burned, and now you presently have no way of avoiding feeling the harsh physical bodily pain of the burns right now, then that pain and you feeling it right now (and in the near future) is part of unchangeable eternal reality, not a matter of your choice. Typically, all bodily feelings such as pain, hunger, and fear all fall into this category. Typically speaking, you in your present are no more presently choosing to presently feel those feelings then you are choosing to beat your own heart at this exact moment. The irony is you may have significant influence over the heart beats and feeling of others in the future, including but not limited to your so-called future self. All you have to do is stab someone in their heart, and it will stop. But just as a knife cannot cut itself, you have ironically much less influence if any at all over your so-called own heart in your own present. If you start stabbing the human body you call yours, it is the future slightly older version of that human, not the present one, whose heart will stop. Physically, when you feel the pain, the metaphorical knife that cuts you is not the same as the knife in your hand, and your choice in such matters of physics is generally limited to whether or not to cut others like you have been cut, versus rebelliously breaking the cycle, which is what some would call a karmic cycle. Such breaking of such cycles is an example of what I would call free-spirited rebelliousness, with the clarification that I use the terms self-discipline and spiritual freedom interchangeably
With the spiritual, the opposite is true: The metaphorical knife that would kill your inner peace or not is always only in your own metaphorical hand, meaning under your control. Nobody can steal your inner peace, and nobody can force inner peace upon you, because the 'inner' in inner peace refers to specifically you, meaning the real you, and what the real you really controls (i.e. your choices). It is on you whether you choose to fully and unconditionally accept the unchangeable. You are 100% in control of your choices.
Like the past as a whole, you in a sense may choose your inner attitude about unchangeable things such as unavoidable (i.e. outer) feelings like pain, fear, hunger, discomfort, and anger. For example, if you are feeling unavoidable pain or fear right now, you can choose to add the negativity of resentment to the situation instead of fully and unconditionally accepting the unavoidable and unchangeable. In this way of using the words, it is only an aspect of your 'inner' attitude if it is something you presently control (i.e. choose) in your present. In the book, sometimes phrases like "bodily feelings" and "urges of the primitive flesh" are used to describe these past-like aspects of the outer present that are describing things occurring inside the human skin. They happen to be happening inside the human skin (which is an irrelevant moot fact of happenstance), but they are not spiritually inside in the sense of inner peace and the sense of the real you, meaning in the sense of your present conscious will and what you are consciously choosing (a.k.a. controlling) in your present.
Everything that exists in your past and outer present (and unavoidable aspects of the future) are like a movie that you have to watch but cannot change. You are not the director or writer; That movie has already been written, directed, and recorded into eternity. However, you can change your inner attitude about it, even though that won't change it. The unchangeable won't change because you changed your attitude about it, neither for the better nor worse, which is all the more reason to choose love over hate, acceptance over resentment, and allowance of the unchangeable over fighting to change the unchangeable.
Before you could lift a finger to change the way your so-called present is right now, it would already become the past. Thus, the present you are presented with is essentially already the past. Fully and unconditionally accept it. Fully and unconditionally allow it. Fully and unconditionally let go of all resentment, unforgiveness, or hate for it. Let go of any fighting against it or fighting to change it (since it is unchangeable). These instructions are not merely a path to inner peace; they are inner peace. If I had to define inner peace in six words, I would define it as not fighting to change the unchangeable. That is not merely how you get inner peace; that's what inner peace is.
Learn to fully and unconditionally accept, allow, and love what is.
What of the future, you might ask. You may be able to consciously choose aspects of what will be. And that's great because when it comes to your choices you always get exactly what you want, meaning what you choose. When it comes to your choices in your present, you are dealing with a blank canvass or a sandbox. You get exactly what you choose, so it becomes even more irrational to resent it. The majority of this post has focused on the fact that it is irrational to say things like "the past 'should' be different than it is" or otherwise resent the unchangeable (e.g. the past) for being the way it is; But it is even more irrational and absurd to say something like, "I have the choice between X and Y, and I 'should' be choosing X but I am choosing Y."
As irrational as it is to fight to change that which you know cannot be changed, it is even more irrational to say, "I can do A or B, and I 'ought' to be doing B, but I am doing A."
In relation to the irrationality of failing to accept what you cannot control, it is even more irrational to not be lovingly accepting of your choices in your present precisely because you do control those. And in a sense that's all you control: your choices.
The rest is what it is. It is unchangeable, meaning it is inexorably part of eternal reality.
There is no Is-Ought Problem because there is no 'ought'. It's irrational nonsense. Gibberish, at best.
Moreover, if we consider the future, we can go all the way with it: Then we can alternatively take a step back and consider reality as an unchanging timeless whole. That unchanging timeless whole includes the combination of both (1) what you in your present choose will be, combined with (2) all that already is, meaning all that unchangeably is. It includes the combination of everything that is not a matter of your present choice, along with everything that you choose right here right now in your present.
Having inner peace--which some might call "true happiness" or "grace" or "nirvana" or "being spiritually awake"--is in large part to look at unchanging holistic timeless reality, meaning look at the unchanging eternal whole of all reality including past, present, and future, and to see it as beautiful, lovable, wonderful, and acceptance-worthy. Not evil. Not bad. But, rather, a perfect lovable beautiful harmony.
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"The mind is a wonderful servant but a terrible master."
I believe spiritual freedom (a.k.a. self-discipline) manifests as bravery, confidence, grace, honesty, love, and inner peace.