Baffled about dualism

I am curious, people say the physical is more than likely there. We play tennis in it. We navigate from one place to another in it. We seemingly have to function within.

We have no proof of knowledge of functioning before or after life in this physical world. We have no other access to knowledge of consciousness other than through this world. And yet we claim it is somehow separate from this world (mind and body duality) when we have zero evidence from either outside or inside this reality.

How can we claim any sort of duality other than the very body that does the mind?