In recent years, several research groups have been able to infer the contents of subjects' thoughts from fMRI scans. E-commerce sites are tracking customers' purchases and making ever better predictions about what people will buy. What are the prospects for such technology to be widely used? Are there fundamental technical limitations? We may readily imagine dystopian scenarios for such technology, where privacy as we have known it is no longer meaningful, and the powerful monitor the thoughts of everyone else.…