My subjective world was born with me. I know of no other world and all other persons appear in my subjective world. In this subjective world I have been taught language, which include concepts, such as "physical" and "spiritual". I have been taught to refer to "things" and "objects" and "phycical" stuff. This language has a long history. It was not created in my subjective world. It was taught to me. It exists and has existed in countless subjective worlds and has evolved over time (another concept).
So words like "physical", "mental", "spiritual" have a history. Their meanings have changed and evolved over time. They are signpoints, and they point to subjective experience in a subjective world. The same is true for all other terms, such as "mind" and more importantly "brain". The brain is a concept, an idea that exists in my mind. If someone's skull is opened to me, I can point a finger to my interpreted sense experience and say that is "a brain".
"The brain" is a label and an abstraction. And it points to subjective experience. No two things are alike. Our minds create abstractions so we can refer to two similar objects as both belonging to one class. But the class is never experienced. Only those things which gives rise to the mind made class.
Today, thanks to neuroscience, the idea of brain is far more complex than it used to be. It has greater explanatory power. But it is still an idea that exists in my mind, your mind etc...
But wait, isn't there an objective world "out there" that just "is", independent of my mind? And in this world there are physical things such as brains? A world that evolved over 13,7 billion years to produce me and you?
I think there is an objective world. And I can both know, and not know it at the same time.
How is it that I cannot know it? As Kant pointed out, I cannot know the thing itself, only how it appears to me. But this goes further than many seem to think. Let me provide an analogy:
Suppose you are playing Halo4, and in the game you are controlling a character that is in front of a computer screen, and on that screen this text appears. Then you turn around 180 degrees in the game world and no shred of the computer or screen appears in front of you. Is there a computer behind you in the game world?
No, there is not.
There is an objective reality that gives rise to that experience of the computer screen when you are turned towards it. That reality we happen to know is code, running on the game console. And when you have given the right input to the console, lo and behold, the computer screen appears. You "turn" in the computer world. If others play the game online with you, they are going to have similar experiences because there is really an objective reality that gives rise to the individual experiences of the Halo4 world.
In other words, you can look at a world of trees, flowers and covenant soldiers, then turn right around and this world ceases to exist. Only what you see at any given time exists in any form remotely like what you see. Only the underlying code has objective existence in the Hal4 gameworld.
Now we know of the code and the xbox because we have information from outside the gameworld, from the creators of the gameworld. But suppose you were born into the Halo4-world of the xbox. That is, your subjective experience began to exist within this game.
Could you ever, even in principle, know the code or the console running the code? Or could you only know the code as it appears in rendered form on the hardware? And is there not a world of difference between the two?
In the same way, when you are in a forest and see/hear a tree fall, there is a noise. There is sound. There are soundwaves. What if no one is there? Is there sound? No. Are there soundwaves? No. Is there "a tree falling" there? No. Does something happen in the objective world that would be interpreted as sound, and soundwaves, and a tree falling, if someone was there to experience it? Yes.
But I also stated that I can know and cannot know the objective world at the same time. How is it that I can know the objective world if I am trapped in this sensory subjective world?
Well, by the same analogy, suppose I was born into the world of Halo4:
I could not know the code or the xbox running the code as concepts in my mind. I could not understand them. However I and my experience of the subjective Halo4 world is what the code *is doing on the xbox* (I and my world are not separate, the code is doing both). I literally AM the running code.
So in reality, both the little me and the world of experience that appears within consciousness with the little me, is what the objective world (what would be the code if this were a program) is doing here. It is also doing you and your experience where you are. In that sense, it is very real.
Materialism is the idea that my subjective world of experience is the real world, and so I can point to processes I see within it, such as brains, and say “that gives rise to consciousness”. That is maya, illusion.