From ThornsWiki

Spirits are nonphysical manifestations of natural forces or objects. The belief in spirits is almost exclusive to humans and wicks, though some galdori postulate as to their existence.

Natural phenomena such as changes in the weather, low or high crop yields and natural disasters are attributed to spirits, and humans sometimes make small sacrifices to appease them. These sacrifices are often something symbolic, such as fasting or giving up a particular food group.

Wicks have several myths and legends alluding to spirit worship, though in most tribes belief in the spirits has been phased out. Strong belief remains only among the older generations of wicks. Humans, however, are very likely to have elements of spirit worship in their personal religious beliefs.

In more academic circles, the belief in spirits is regarded with disdain. The Resistance in particular discourage spirit belief, claiming it ties humans to the ancient superstition and ignorance which resulted in their unfortunate circumstances.