This is a brief summary of the most well-known positions in philosophy of mind.
There are two kinds of "stuff". Mind and matter. Often associated with the "soul" hypothesis.
Mind is very different from matter. For example, thoughts do not have length, width, and height (your headache does not have physical dimensions), but matter, and specifically brain activity, does have length, width, and height. If two things have different properties, then they are not the same thing.
How can an immaterial mind interact with the material body?
There is one kind of stuff: matter. Mind is composed of matter.
The success of science in explaining everything else in terms of matter/energy.
Can the properties of mind (conscious experience, and that thoughts are about other things) be explained in terms of just particles moving around?
There is one kind of stuff: mind. Matter is composed of mind
Matter cannot be conceived as unconceived (it is a logical contradiction). Therefore, matter cannot exist without mind.
Do mountains, fish, and so on really not exist when we are not perceiving them?