A notion used to test and refine causal theories of perception, action, meaning, and memory. Suppose it is suggested that for me to remember an event it is enough (a) that I witnessed it, and (b) that this was the original cause of my present thought about it. Then a deviant causal chain might be that I witnessed the event, because of that wrote about it in my diary, and on now reading the diary think about the event. A causal chain is in place, but it is not enough to establish that I remember the event; it is consistent with this story that I have forgotten it entirely. Ingenious deviant causal chains include objects causing hallucinations that match the appearance of the object, thoughts causing spasms causing events that match those that would have occurred had the thought given rise to an intention to act, and many others.