Much recent neuroscientific work, and in particular the programme initiated by Benjamin Libet, seeks to show “the causal closure of the physical”—that mental events never cause physical events, and in particular that our intentions never cause brain events and thereby our intentional bodily actions. But no one is justified in believing any scientific theory unless they are justified in believing that it successfully predicts certain events. Someone is justified in believing that certain events predicted by some theory did occur, if they apparently remember having perceived these events or if some other scientist apparently testifies that they have perceived these events. But we believe our apparent memories of our past perceptions of events because we believe that perceiving those events has caused brain events which have caused our present apparent memories of them; and we believe the apparent testimony of others because we believe that their intentions to testify have caused brain events in them which in turn have caused the words of their testimony to come out of their mouths. So someone could only justifiably believe the theory that mental events never cause physical events if they believe that either their past perceptions or the intentions of other scientists to tell them what they perceived, both of which are kinds of mental events, have caused brain events, which are physical events. So that theory is self-defeating; no one could ever be justified in believing it, or more generally be justified in believing the theory of the causal closure of the physical.