During the later Middle Ages, a number of religiously oriented people behaved in ways that we would consider unusual, yet it was unusual for them to be regarded as mentally disordered. This article reviews late medieval thinking and practice with regard to mental disorder and also with regard to the discernment of spirits, that is, how it could be decided whether an experience or impulse to do something was the consequence of God or a good spirit, an evil spirit, or some purely human cause. Many of the criteria for discerning a good spirit were behavioral, for example, consistently showing humility and discretion, and were clearly distinct from those displayed in mental disorder. A comparison of the criteria for mental disorder with those used to discern spirits shows how the distinction between mental disorder and religious experience could have been made and why confusion of the two seems to have been rare.