In support of this, Chalmers is famous for his commitment to the logical (though, importantly, not natural) possibility of philosophical zombies. These zombies, unlike the zombie of popular fiction, are complete physical duplicates of human beings, lacking only qualitative experience. Chalmers argues that since such zombies are conceivable to us, they must therefore be logically possible. Since they are logically possible, then qualia and sentience are not fully explained by physical properties alone; the facts about them are further facts. Instead, Chalmers argues that consciousness is a fundamental property ontologically autonomous of any known (or even possible) physical properties, and that there may be lawlike rules which he terms "psychophysical laws" that determine which physical systems are associated with which types of qualia. He further speculates that all information-bearing systems may be conscious, leading him to entertain the possibility of conscious thermostats and a qualified panpsychism he calls panprotopsychism. Chalmers maintains a formal agnosticism on the issue, even conceding that the viability of panpsychism places him at odds with the majority of his contemporaries. According to Chalmers, his arguments are similar to a line of thought that goes back to Leibniz's 1714 "mill" argument; the first substantial use of philosophical "zombie" terminology may be Robert Kirk's 1974 "Zombies vs. Materialists".