  Physical properties are public, in the sense that they are, in principle, equally observable by anyone. Some physical properties – like those of an electron – are not directly observable at all, but they are equally available to all, to the same degree, with scientific equipment and techniques. The same is not true of mental properties. I may be able to tell that you are in pain by your behaviour, but only you can feel it directly. Similarly, you just know how something looks to you, and I can only surmise. Conscious mental events are private to the subject, who has a privileged access to them of a kind no-one has to the physical.