Idealism suggests that there is the mind only. If this being the case then the world (things, space, and time) could and would be known without the senses (eyes, ears, nose, mouth, and skin). Clearly, to know the world we need the senses and also a capacity for thought – the mind. Berkeley argued for a reason-only position. Again, the same problem can be pointed out against him or her.
Bu the materialist position is not any better. If there is only matter then how do account for knowledge? Obviously, we need the senses and reasoning of sense-experiences. Here, Kant is correct that we need to combine knowledge from the senses with interpretation from reasoning. But the reasoning should lead us to ontological question again of what exists, not only that we have only phenomena (representations) to work with. Ignoring what actually exists is to live a life not fully engaged with the world.
This is why I bring up a game of tennis as example. Tennis players, umpires, spectators all know the qualities and properties of the world and engage in competition, judgement and enjoyment of it. Without this trust of the world we would be forever wondering if the laws of the game would change, or will the ball disappear and reappear elsewhere on the court. And that would not be enjoyable for anyone involved.
The things and qualities of the world have been consistent in my life time. And my lifetime is not a particularly special or different one to any other person’s in any time or place. And if it is different, then, that is the reality, and that this difference must be accounted for. What remains is the thing, the body. What cannot be account for is the mind, soul, or spirit. If it not there to begin with, it wasn’t there then either.
And for this reason I argue the material monist position. What is called, in the least, the mind, soul, and spirit supervenes to the body. In all likelihood the mind, soul, and spirit does not exist, that is, they are illusions.