Xiaogu (小故) refers to an insufficient cause or condition. The Mohists categorized the causes of things being so into dagu (大故major cause) and xiaogu (minor cause). It is uncertain whether xiaogu leads to the corresponding consequence, but it is certain that its absence leads not to the consequence. In simpler terms, xiaogu is a necessary but not sufficient condition for the occurrence of events.

With the minor cause, events are not bound to occur; without it, events definitely will not occur. (Mozi)