One prominent contemporary argument for dualism starts with the premise that you have a special ways of knowing about your own mental states. This knowledge is not usually based on any evidence or observation. You can just tell that you're thinking about elephants. You don't have to infer it from any evidence. But other people do have to infer what you're thinking, from your behavior and from what you say. So you're in a better position to know what you're thinking than they are. You have a kind of special or privileged access to your mental states which other people lack.
You don't have any privileged access of that sort to your height or weight or shoe size. In principle, other people could be in a better position to know your height than you are.
Most philosophers will agree that we do have some kind of privileged access to our own mental states, even if it is difficult to spell that claim out in detail.
But some philosophers go further and argue that the only way we can explain or account for this privileged access is if facts about your mind are facts about what's happening in some private non-physical realm. Physical objects and their physical properties, on the other hand, are all publicly accessible. Other people can know more about your height, and your weight, and the physical state of your brain, than you know. So if your mental states were just physical states of some sort, like neurophysiological states of your brain, then that would make your mental states all publicly accessible.
(See Phil 3 Notes on Arguments from Privileged Access and Inverted Spectrums.)