Most people if you talk to them are not actually dualistic. They separate the physical from the psychological, the psychological from the mental, and the mental from the spiritual. We all draw these lines differently and often use different words, and some of us dismiss certain categories entirely, yet explicitly. For example, if I say I don't believe in an eternal soul or anything like it, and I'm convinced it's "biology all the way down" and consciousness is an illusion, then I am still separating the spiritual out in order to deny it.
Psychologists differ from most of the public in their willingness to explore and question and expand on these boundaries. And I think we often put the lines in different places. For many people, "mind over matter" is sufficient detail when struggling to push through the struggle. And if that works, it's valid enough. For others, to distinguish the psychological from the mental would be bizarre and artificial. They could even cite evidence and dissect it. But overall I think psychologists have a deeper and more nuanced sense about the spectrum or spectra that live where most people see monism or dualism or simply religion.
Brain activity is physical. To say about any activity that it "physically changes your brain" is borderline meaningless. If you remember something, your brain has been permanently altered. Even if you learn it and forget it, your brain has been permanently altered. For that matter, I doubt you can experience anything at all without some permanent alterations, whether you form a memory or not. Our candles burn down. The clock ticks. All known experience is accompanied by brain activity without which it would not transpire. Your most spiritual experiences are - probably - almost certainly - also physical. If you had enough and small enough and advanced and coordinated enough nanobots swarming your system, any of your subjective experiences could be tweaked, and so could, in all likeliness, any of your decisions. It isn't unlikely that the physical process by which you make conscious, free decisions could be tapped into, and your free decisions directed externally. The irony is that if this were true, those decisions might be about as free as they were before.
Most people would regard that view as monistic: it has replaced all the mental and spiritual questions with physical ones, or, at least, it hypothesizes that in the end, the hierarchy is flat and they all amount to the same reality, which objectively exists, with physical extent.
You can believe it's "biology all the way down" and the spiritual is all ultimately part of physics and still look at life and see these: physical, psychological, mental, and spiritual. Because you might disagree with me on those, and on their boundaries, I'll tell you, for the sake of discussion, what I see this way of slicing up the pie to mean. It's maybe informal. I'm not even sure that this is how I usually divvy up. But I am confident that I think obsessively around this topic and examine it more closely than most people you will meet.
The physical would be something like a wart on your foot. It's caused by a virus, which itself barely qualifies as living (if at all) and is widely regarded as possessing no mind, consciousness, or will. If you are not conscious of the wart, it is still there. If you are conscious of the wart, it is also there. That the wart might upset you or make you laugh or change your behavior, or that your behavior might include putting on a wart removal patch or going to get it cut or frozen off, does not make the wart a psychological entity itself, or any less physical. In this aspect, the wart is largely like a rock or a distant galaxy. Even if thinking about the wart differently could get it to go away, that would be through immune system activity acting on a physically real viral infection and eradicating it. Now, I could have started with rocks, but I wanted a more illustrative example. A wart lives in living tissue and is itself alive. However, it is fully physical and not psychological in nature. Looking at the moon and wondering at its beauty does not make the moon psychological, and the same goes for the ugliness of a wart. Most medical questions are physical, unless they are about behavior, emotions, communication, etc.
The psychological level, then, you will notice, is contained in the physical, but separated from it by practice. For a parallel example, vegetation would include fruits and nuts, but when we say vegetation, we typically don't mean fruits and nuts. Similarly, perhaps nuts are themselves fruits (it depends how you define both), but when we say fruits we don't mean nuts, and when we say nuts we don't mean fruits. When we say movement, we specifically mean something a little unlike a TNT explosion. If it were a TNT explosion, we'd call it an explosion or a blast. If we say it's a movement, even though a blast really is movement, we are pretty strongly implying an action other than a blast. It's worth going into detail about levels of specificity and the exclusions we imply when we use them, and it happens that this (mostly) is how I see the four categories I've mentioned. It's worth putting in seams.
Psychological versus physical is important in practice, because what's physical generally doesn't change just by noticing it or thinking about it. They are not exactly mutually exclusive (and one, I'd personally argue, is fully contained in the other, though I'd leave a little room for surprises on that front), but they are qualitatively and practically different. For example, motivation is psychological. By one definition, without motivation, you will not do anything: if you do anything, motivation is always part of the equation, as motivation is that inner fire that nudges or persuades us to move at all. We may influence motivation different ways, but we do not ultimately control it directly, nor do we exactly bring it into being or know how it works. Motivation exists before us, before what we understand as our identities and choices. We can shape it but not stipulate it. If I am not motivated to get up and go out for tacos, then, by one definition of motivation, there is no way I will do so. Pushing or being pushed to go, to move from a state of no motivation to acting, would involve the introduction of some form of motivation. Without the motivation, no motion.
In this sense, heartbeat is physical - it happens however we think about it, though we can intervene and influence it, much as we could choose to move a rock, or knock one aside while carrying a sofa through the garage door. Heartbeat is physical, but dreams at night, shared memories, humor, jealousy, anger, pride, anxiety, care, romantic love - these are psychological. They are primarily mental phenomena that are however somewhat spontaneous and somewhat outside of direct conscious control. This is the realm of feelings, but also of much of mental illness, and an underpinning of top performance in any field.
There is also, I would argue, the mental. This is the level of engagement with thought that is at least somewhat distinguished from feelings. For example, searching for the solution to a sofa stuck in the garage door, or for a mathematical proof, is a consciously initiated process that most of us would accept operates outside the realm of feelings. It doesn't matter whether you are upset or assured - a solution will either work or not, on its own merits. This would seem to harken back to the physical, but then, let's not forget, so does the psychological - we've already claimed these are all somewhat like babushka dolls.
Finally, many of us would claim there's a spiritual level. This is the level, perhaps, of consciousness itself, or, if you believe in it, of free will, or of the eternal soul. It's what's heightened by meditation or art or an epiphany or the birth of a child or loss of an elder. It is of course related to mentation, feelings, and the atom-ridden brain tissue that may support all these other levels. It's the level of responsibility, determination, higher love, mind over matter, good and evil, compassion, and so on. These all tie into the other levels, or even derive fully from them, but they take on another significance at the spiritual level.
This isn't an original division at all. All I'm really doing is personally codifying the pieces of a popular tarot spread: body, heart, mind, soul. These of course are not original to the tarot world either. I don't know who came up with them. But I do think it's a useful way to put seams in the map.
More to the point, I believe psychology is often about, and psychologists often preoccupy themselves with, these distinctions. That is, maybe I can't fully control my motivation to get out of bed rather than hit snooze, but it can certainly be instructive to say: this is a mind problem, not so much a mechanics problem. I can get out of bed, joules-wise. I know how not to hit a button with my hand. Though my motivation has a physical basis, the issue isn't much like the sculpture outside the building that no one person is bulked enough to rip out by hand. This isn't about rudimentary skill or raw activation energy. It's psychological and therefore probably susceptible to influence by the mental and the spiritual. It's liable to change from day to day or depend on how I interpret the issue. The problem may still be incredibly difficult. It could prove more difficult to solve than many purely physical problems. But it's useful to categorize it as psychological foremost, and then dig into what details you can uncover.
I invite you to look at any problem you have, big or small - and maybe the bigger one is more fun - and examine it from each lens: physical, psychological, mental, and spiritual. I think you'll find they're all relevant, and you'll probably feel that one is the most appropriate category. But after putting it in a bin, don't forget the others. Every human problem has tendrils in each one of these regions, and we solve the tangle by investigating and experimenting.
The most difficult problems are usually - not always - best approached from multiple defensible angles, rather than going all in on one attack at a time. For example, the snooze problem might dissolve after steps taken on each of the four levels above. Or a heart problem might be neutralized with two or three dozen little health and lifestyle changes that, together, are more powerful than any single known step.
