Recently, many philosophers and psychologists have claimed that the explanation that grounds both passivity phenomena in the cognitive domain and passivity phenomena that occur with respect to overt actions is, along broad lines, the same. Furthermore, they claim that the best account we have of such phenomena in both scenarios is the "comparator" account. However, there are reasons to doubt whether the comparator model can be exported from the realm of overt actions to the cognitive domain in general.There is a lingering worry concerning such explanations of thought insertion: the "What is compared to what?" problem. Here I examine two ways to tackle this problem. First: thought insertion consists of the misattribution of strings of inner speech which are not attenuated (thought insertion is thus another name for auditory verbal hallucinations).Second: thought insertion is misattributed inner speech which exhibits the same phenomenological characteristics as normal inner speech. After explaining the types of problem that each of these potential solutions faces, I conclude with a set of open questions that the comparator theorist has to tackle. some version of the "comparator" account, forcefully put forward by Frith (see, e.g., Frith, 1992). Frith's theory draws on the corollary discharge model of perception proposed by Helmholtz (1860), and extended by von Holst & Mittelstaedt (1950) and Sperry (1950) to deal with motor acts. The model works in the following way: whenever a motor command is issued, the brain predicts, based on an efference copy (or corollary discharge) and the work of some "forward models", what proprioceptive and sensory feedback will ensue. If the prediction matches the actual feedback received, that feedback is attenuated and perceived as generated by the subject: the subject gets the feeling that the action is his own. If the incoming feedback signal that results from the motor act was not adequately predicted, it is felt in its full intensity and the subject does not get the sense of being the agent responsible for the movement. Frith's idea is that what goes wrong in passivity phenomena in general is that the forward models which issue predictions malfunction.There are now several versions of the comparator account, as well as an increasing suspicion that it requires some sophistication or to be complemented by other accounts (see, e.g.