 According to the object-based conception, for example if paradigmatic physical objects such as rocks or planets are solid or are located in space or have mass, then the property of being solid or located in space or having mass is a physical property. (The best examples of philosophers who operate with the object-conception of the physical are Meehl and Sellars 1956 and Feigl 1967; more recent defense is to be found in Jackson 1998.)